What is are force?

Force is any push or pull that changes or tends to change the state of rest or uniform motion of a body.

F = ma (Newton's Second Law) | SI unit: Newton (N) = 1 kg·m/s²

Four fundamental forces of nature:

1. Gravitational
2. Electromagnetic
3. Strong Nuclear
4. Weak Nuclear

Sir Isaac Newton formally defined force in Principia Mathematica (1687).

Force in one word: Push or Pull (technically: "Interaction").