Mouse Hole

The cartoon mouse's refuge: a neat arch-shaped hole cut into a wall at the floor, sometimes with a door.

If you have one of these in your house, expect your perfectly triangular cheese wedges to go missing soon.

Often the gate to Mouse World.

Examples of Mouse Hole include:

Film

A rare live-action example, though more realistic looking, can be found in the movie Mouse Hunt.
It also appears in Willard, created by the giant rat Ben chewing through the wall.

Episode 20 of Monty Python's Flying Circus has a ratcatcher looking for rat holes in a house. The first one he finds is a Mouse Hole, complete with fake mice on a string.
There is a mouse hole in Mr. Bean's apartment. In the Christmas Episode, he puts a mousetrap in front with a present on it.

A trademark schtick of Garfield strips. Sometimes this even extends to windows with pots of flowers under them, mailboxes, welcome mats, etc.
Ziggy

While they're not mice, the Minish in The Legend of Zelda Minish Cap have several small holes that look like this. They do, indeed, lead to the Minish's Mouse World.
In Luigis Mansion, these are all over the place. Some release golden mice, worth major dollars. Luigi can enter most or all with a special device, usually yielding more treasure. Or ghosts. Often both.
One of the first puzzles of Sam and Max Hit The Road requires you to grab a stash of cash from one of these.

Tom and Jerry, as in the picture above.
Early Mickey Mouse cartoons didn't show his house, but early comic strips and children's books did. It had a round, obviously mousehole-like front entrance despite Mickey's large, non-mouselike size.
- In 1931, Mickey's house began to be shown in the cartoons and had a normal front door there, after which the ancillary material was retconned to match.
The "Pixie & Dixie and Mr. Jinks" shorts on The Huckleberry Hound Show
Likewise the Looney Tunes cartoons featuring Speedy Gonzales and, before that, Babbit and Ratstello.
- Sniffles lived in a mouse hole in some shorts as well, even earlier.
Mostly averted in Walt Disney's Cinderella, but there is one.
My Little Pony Friendship Is Magic: Fluttershy may have purposefully installed a mouse hole in her house to shelter mice.
Invoked in Tex Avery's King-Size Canary, when the jumbo-sized cat and mouse do this with a train tunnel (@ 7:02).
You might notice the classic arch-shaped mouse holes in the floor boards in the background on The Simpsons.