Veteran Unit

A variant of Character Levels for Strategy games; after multiple kills a single unit "levels up" and becomes more powerful. It tends to be integrated into the backstory a little by suggesting they are more experienced (hence the "veteran" designation) or gained a higher army rank.

As RTS games tend to focus on larger scale tactics rather than individual units the effects can vary a lot; sometimes the difference is hardly noticeable and other times they become so much better they are practically a new unit. The amount of possible level ups also varies very heavily between games. Some games even have ranks for kills without it actually changing unit stats at all. When implemented this is done to promote keeping your units alive rather than taking a We Have Reserves approach, and tends to support Video Game Caring Potential as a result.

If a character/unit is persistent over multiple missions but loses the veteran rank between missions, it can be a form of Bag of Spilling/Reinventing the Wheel. If taken far enough, units end up gaining a Character Level or transforming into a Prestige Class rather than a rank.

Examples of Veteran Unit include:

Tabletop Games

Star Fleet Battles supplement J1 Fighters: Pilots started out at Green quality and gain points for certain accomplishments, including destroying enemy ships. A pilot with 10 points becomes Good, and one with 50 points is an Ace. Each higher rank gives bonuses in combat.
Traveller Classic supplement Book 4 Mercenary: Military personnel gain experience points for various events, such as passing basic training, number of terms served, being decorated and coming under fire during a mission. A character's morale rating is equal to his number of experience points.
Only War has Regimental Awards/Honours (bonuses for the whole Squad, representing experience in fighting certain type of enemies and/or morale effects of being commended or renowned for certain achievements), as well as Veteran Talents (individual advancements unlocked by achievements, often by the whole Squad).
Shogi has a rule where a piece can be promoted by being moved to the farthest three rows on the board, upgrading it such that it almost becomes a new piece. As some variants of Shogi predate Chess (which has a somewhat similar rule), it is likely the Trope Maker (with Command & Conquer as the Trope Codifier for games).
As noted above, Chess generally has a rule where a pawn can be promoted to be any other piece but a king if it reaches the other end of the board.

Video Games

Command & Conquer varies this trope quite a lot, in some of the games it isn't there or makes barely any difference, in other games keeping veterans alive can vastly increase the strength of an army.
- The first two games (Tiberium Dawn and Red Alert) did not feature any type of veterancy. Tiberian Sun was the first in the series to feature veterancy, but gaining veterancy took a lot of kills and the bonuses weren't noticeable.
- For Red Alert 2, they went to the extreme and units got far more powerful. Some top veteran units became very near One-Man Army levels of power.
- This was dialed down some in Generals, but veterancy could still handily make or break a battlefield clash.
- Tiberium Wars saw the veterancy system dialed down a bit more, but given that it has not fluctuated much in Red Alert 3 or Tiberium Twilight, it seems that the developers have found their desired balance.
Total Annihilation uses this but tends to be quite subtle with it. Kingdoms is more extreme with it and some units become much more powerful after some kills.
Warzone 2100 is one of the most extreme examples. There are many ranks (although the differences are subtle). In the single-player campaign, veterans are persistent (along with everything else) and can be transferred to new vehicles if they stay alive. Keeping veterans alive throughout is almost essential to keep up with the technologically superior enemies later on in the game.
Both LotR: Battle for Middle-Earth games have this mechanic. One of the possible purchased upgrades for a unit allows it to instantly jump to the second level (out of possible five).
In StarCraft, units increase in ranks for kills but it makes no difference to their stats.
Myth: Every time a soldier makes a kill, it gets a little more deadly, and survivors carry over to the next level. It is critical on some levels to have veterans from the previous levels.
Some games in the Nintendo Wars series do this.
Any troop that lives long enough to learn how to shoot straight in X-COM becomes a veteran, and you do become attached to them
The valor stat in Total War arguably counts, as the unit's stats and morale increase with every point. This led to the infamous "Jedi generals" in the first Medieval as their high stats, paired with the high valor typically gained by a unit getting whittled down in battle, effectively led to one-man armies.
Halo Wars values experience to Stars that pop above a unit's health bar. These stars give a small boost to a unit's fire rate and DPS. Most units get up to three stars except for certain situations. Fully upgraded Spartans in a super-unit (Grizzly or Hawk) with full experience gives a whopping 14 stars! On Covenant leaders, their number of stars also increases their special ability's damage. A DLC game mode called Reinforcements can show the effects of an army with a lot of stars very well.
In World in Conflict, individual units that survive long enough while killing enemy units and capturing points gain up to four levels. Leveled-up units (particularly the fourth-level "general" ones) gain considerable boosts to movement speed, rate of fire and special ability recharge times.
In the Civilization games, units that gain combat experience do not become more powerful, but their health bar becomes bigger, meaning they will last longer in battle. Within the game's dice roll combat system this means they get more chances to score a critical hit even against superior units.
- In Civ IV and V they do advance in level, obtaining promotions that make them stronger units.
EndWar: Units that survive enough battles become better and can be upgraded.
In S.W.I.N.E., units gain experience from dealing damage to the enemies and can acquire "veteran", later "elite" status that improves their stats. It also changes their looks, adding more bling and kill markings while also making the unit look more used and worn-out.
Battle for Wesnoth has this feature in the extreme; although it is not an RPG, every unit can gain 1 to 3 levels, each one entailing a transformation into a new unit class. The effects vary from a simple increase in power, to the addition of new abilities, to a complete transformation of the unit's role. Building up veteran armies is critical in campaigns, and even in multiplayer games (which are much shorter), managing your xp well to gain level-ups can significantly impact a game.
Earth2150 and Moon Project feature a veterancy system that slowly improves a unit's capabilities over time rather than having predefined veterancy levels. The bonus effects can be improved by adding a flag or banner to a nearby unit. This can make a substantial late game difference, especially since a meticulous player can carry the same units from mission to mission.
Company of Heroes has a lot of variations on this: all four factions implement veterancy differently. Armored units visibly gain upgrades like sandbagged protection or extra armor plates welded on. The Americans have the usual combat experience scheme wherein killing enemy units gains them ranks. The Panzer Elite have something similar, only instead of gaining general stats they have the option to raise their offensive or defensive statistics. British units don't gain veterancy directly: instead, attached Officer units gain experience, and units in an aura around them benefit. The Wehrmacht don't gain veterancy through the normal way at all; instead, they must purchase veterancy at a rather hefty price.
In Pharaoh your military units start as "Green" and become "Regular" once and if they go through your war academy. They gain experience and levels the more they fight, becoming more deadly and resilient. In some scenarios it's vital that you level-up your units as you can only field six regiments since the enemy comes in overwhelming odds and some units such as war chariots literally stomp normal units.
In Warhammer Dark Omen the regiments gain experience levels and become better warriors and wiser wizards by slaying foes.
Advanced Strategic Command has units gain XP by fighting, the level is indicated and noticeably affects performance (as demonstrated in the infantry tutorial). The latest (in 2016-2017) development fork deals with splitting experience to defense and offense tracks.