Instant Thunder

FLASH! Lightning streaks across the sky! A few seconds later, the thunder arrives. BOOM!

In Real Life, thunder is the sound caused by a lightning strike. It has a delay because sound travels slower than light, and so the farther away the lightning strikes, the longer it takes for the thunder to arrive. If you hear thunder at the same time of a lightning strike, it probably means that you're inside the eye of storm.

Not so in fiction. Here, lightning almost always arrives with the sound of thunder, no matter where the lightning is. This happens so often that it is remarkable when thunder does have a delay. Expect quite a few aversions to mention counting the seconds to see how far away the lightning struck, especially if the target audience is children.

Most writers, when they avert the trope, will generally include the counting to show the distance of the storm.

This is such a universally used trope that only subversions and aversions should be listed as examples.

Related to Dramatic Thunder and Thunder Equals Downpour. Falls under the Rule of Perception.

Examples of Instant Thunder include:

Aversions

The first Poltergeist movie has the big sister teaching the little brother to count. They know the storm is coming closer as they count because the thunder comes quicker.
Averted in Attack of the Clones, with the DVD commentary noting the trope.
The Puzzle Place taught children to count the seconds of delay between the lightning and thunder to find out how far away the storm was.
The children's book Thunder Cake features a child and her grandma working together to gather the ingredients for a cake before the storm comes. During this they are counting the time between the lightning and thunder to figure out the time they have left.
In The Shawshank Redemption, Andy escapes from prison during a thunderstorm; one aspect of this involved him banging on a sewer pipe with a rock during thunder rumbles to drown out the sound. The thunder rumbles were almost at the exact same time as the flashes, from which Andy knew exactly when to strike the sewer pipe.
An unlikely aversion shows up in the 1973 animated TV special, BC: The First Thanksgiving. When a volcano erupts in the background, the resulting sound and shockwave takes several seconds to arrive.
Another non-thunder example is in True Lies. The atomic blast is seen long before we hear its relatively faint rumbling. Understandable as they're at least 12 miles away, which is stated to be the minimum safe distance, though it still took a lot less than a minute for the sound to hit.

Subversions

The Berenstain Bears has counting thunder delay as a key point in The Berenstain Bears Count Their Blessings.