The simplest version is rate-of-change: today's price divided by the price N bars ago. Everything from macd to true-strength-index is a smoothed or normalised variation on the same question.
Momentum has two distinct uses that are often confused. As a trend-confirmation tool it is used in the direction of the move: rising momentum supports continuation. As a reversal tool it is used through divergence, where price makes a new extreme and momentum does not.
Those uses can point opposite ways at the same time, which is why momentum readings are easy to rationalise after the fact. Decide in advance which use your system makes, and remember that momentum is computed from past prices and therefore lags. See indicator-lag.
Related: rate-of-change, divergence, macd, indicator-lag, oscillator