Put simply, a force is something which changes the momentum of an object. The magnitude of a force is the rate of change of momentum caused.
This is shown by manipulation of the famous equation
Of course, because force is the rate of change of momentum, it can be linked to an object whose mass changes while velocity remains constant.
Force can also be related to an object whose mass and velocity change simultaneously, but those equations involve calculus, so I won't include those here.
In physics, a force is any interaction which tends to change the motion of an object. In other words, a force can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate. Force can also be described by intuitive concepts such as a push or a pull.