Momentum happens in many timeframes: today, the next two weeks, months, quarters and years. Momentum means that in each timeframe, we have four kinds of tempo: things we have yet to begin, things we are beginning, things we're making progress on and things we're completing. It's the mix of tempos in timeframes that gives momentum its character.
The power of momentum is how it creates energy that supports our efforts and actions. Without momentum, we have to manufacture energy, which is a Catch-22 because we have to have enough energy in the first place to manufacture energy for effort and action. This is one of the secrets to high performance.