If an iambic poem line has 7 syllables, is it considered trimeter or tetrameter?

Neither, an iamb is a pair of syllables, so it would be 1 too many for trimeter and 1 short of being tetrameter.
There are time when this is acceptable, as in the case of ballad stanza where there are 4 iambs in the first and third lines and 3 iambs in the second and fourth lines of each 4 line stanza. Sometimes the wording will allow the use of seven syllables in the short line, but not in all of them.
dd

A good reference page can be found at this website.
http://www.uncg.edu/~htkirbys/meters.htm
When in doubt, I usually refer to it.