I think of commitment as a steadfast promise/agreement to follow through on something. There are so many things to which one can commit!
If you're up for a little more reading, here are some other related threads on commitment and how people define it for themselves, mono reactions, etc. (maybe they can all be combined):
Commitment: what is it?
Misperceptions of others/how to be candid w/o turning ppl off?
Polyamory, commitment, and emotional investment