If I only attended college for one semester in 2021, was I considered a full time student?

[Hal_Al]

Yes, if that semester covered parts of 5 calendar months, e.g. Jan 30 to May 1 or Aug 31 to Dec 1 and you carried a full time class load per the school's criteria, e.g, 12 semester hours.

But it depends on why you are asking (what tax attribute are trying to qualify for).  If this is about whether you can be claimed as a dependent, it's the 5 month rule that  usually matters. 

[JillS56]

It depends.  If you were considered a full-time student for that semester, yes.  You will have to check with the college to find out what is considered as a full-time student.