In stocks, from what I've read momentum refers to the directional trend. It depends on a time period. If you take the average price from a few samples at the start of the period, and at the end, if the start is greater than the end then it's a downward trend, and they assume there's something resembling physical momentum where it's most likely to continue going down. Momentum in trading is all about the idea of the price following the path of least resistance. Just like you can look at the support and resistance lines and assume those will add friction to the momentum as it gets closer.

The thing is that a chart doesn't obey physical laws. I'm new to the stock market, and I'm not clear on all this stuff, but my first impression is most of this is people fooling themselves. It's like a Ouija board, how if enough traders believe it's going to move a certain way, their actions cause it to happen without them realizing they're the one's doing it. Then they ascribe it to an external force.

I think it'll work sometimes, until it doesn't and behaves completely differently and you lose all the money you made using the momentum strategy.