As
Webster 1913 notes, the Sanskrit word "sutra" has root meanings of "thread". It refers to a
thread of discourse, a line of argument. It also has a relationship with such contemporay English words as "
suture".
As whizkid notes, there are a cast range of literatures of diverse kinds in Indian Sanskrit literature which are referred to by this term. Obvious examples include the Kama sutra of Vatsayayana.
A small adjustment to whizkid's write-up. Pali is not really a language as such. It is the written form of a particular north-east Indian Magadhan dialect spoken at the time of the Buddha. "Pali" in fact just means "written". It is somewhat awkward and un-nuanced compared to Sanskrit. For example, the word for "body" means "raw meat" and that for "mind" means "not with raw meat".