I know many people (chiefly S'faradim) don't have the custom altogether of refraining from eating kitniyos on Pesach. Among those who do have such a custom, moreover, there are varying customs as to what is forbidden as kitniyos: I've heard some people refrain from eating peanuts while others do not, for example. What foods were originally considered forbidden as kitniyos wherever the ban began?
The earliest source, Sefer Mitzvot ha-katan (SMaK) of R. Yitzchak of Corbeil lists, in French, pois (peas), fole (perhaps, fave, fava beans, 'ful' in Hebrew) & ris (rice), which he lists as 'types of kitniyot'. Prior to that, the designation 'kitniyot' applied primarily in the context of the laws of kilayim. Rambam conceptualizes three categories of crops that grow from the ground: cereal, kitniyot, and vegetables. Kitniyot are the group that occupy that middle space.
According to the Tur's rationale for the kitniyot ban, namely, that some grain might be mixed in, it would make sense for kitniyot to be defined as legumes proper. Legumes have been used for centuries in crop rotation systems, as nitrogen-fixing bacteria reside in their root systems, and they replenish the soil better than any other plants (artificial nitrogen fixing is only about a century old). It would thus be likely that a stray stalk of cereal grain would grow within a field of legumes.