An animation with a framerate of atleast 60 fps.

When in doubt, you can check the metadata of a file yourself by clicking on the » arrow next to the Size of a post under the Information header and look for the Video Frame Rate value.

Step-by-step example

1. Open a post
Danbooru post #3838401

2. Scroll down to the Information section
Information ID: 3838401 Uploader: Etou Date: over X years ago Size: 14.1 MB (960x720) » Source: youtu.be/PcrhnroAHU8 » Rating: General Score: 🡅 XX 🡇 Favorites: XXX Status: Active

3. Click on the » arrow on the end of the "Size" field
Size: 14.1 MB (960x720) »

4. Scroll down until the Track1 section.
...
Track Volume	0.00%
▶ Video Frame Rate	60
X Resolution	72
...

At the bottom of this section you will find the Video Frame Rate value, which should be 60 or higher.

This tag implicates animated (learn more).

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