The above answers regarding the technical meaning of momentum are all correct. I would add that momentum is also used outside of the context of science and math where people use it more conversationally.
One might say for instance that a project is gaining momentum.
This would mean that the project might have started off slow, with only a few people involved, but now it is moving along more quickly and more people have signed on to help. This is also true of political campaigns, kickstarter/crowdfunding/crowdsourcing campaigns anywhere where a critical mass of people is necessary.
Momentum when used in this way is often synonymous with gaining traction.