What is the unit of YGCT in jstat output

The unit is seconds.

YGCT - stands for Young generation garbage collection time

FGCT - stands for Full garbage collection time

Both values sum up the time the GC takes for the specific task.

The unit is second according to https://docs.oracle.com/javase/7/docs/technotes/tools/share/jstat.html.

Is not explicitly stated but it is easy to grasp from the paragraph Using the gcutil option:

The output of this example shows that a young generation collection occurred between the 3rd and 4th sample. The collection took 0.001 seconds and promoted objects ...

0.001 is the difference between 2 sequential YGCT because is about a young generation collection; check the paragraph at the link above in order to learn more about the context.

Accepting seconds as the unit for YGCT while also considering that would be illogical/confusing to have something else for FGCT than one could conclude that seconds is also the unit for FGCT.

UPDATE

jstat for java 8, jstat for java 9, jstat for java 10 and jstat for java 11 all offer the same example but with other values than mentioned here; see The collection took 0.078 seconds paragraph. I stress again that is important to understand what the time measuring is for (YGCT) and why (because is about a young generation collection).