Right, just as LUCIFER mentioned Plautus etcetera: I believe "senarius" was the term when used for the theatre, trimetre (3metra, a metron being a group of two feet) being the name outside the theatre; I've read that the main difference in composition is that senarius was analysed into single feet (so, "6 feet") as opposed to metra, where in metra the rules of variation become much stricter (like variation is technically only allowed in the "first half" of each metron).
[I personally am a big fan of composing in iambic trimetre, and it is a great metre for satire (and I guess the trimetre was a metre for old satire but I personally havn't come across them yet), and translating Horace's 3rd Epode into English iambic distichs was an awesome experience, and can't wait to get around to the others]
Anyways, I realize I pretty much just said what LCF, but again, I like to put my two cents in, in case I covered something passed over