Webster gives us a definition of "force" in war or in card games but not a physical one.

Physicists give us a way to calculate force but there does not appear to be a good definition for what it is. I suppose that "force" can be considered a "primitive" concept that should be understood implicitly, but if we were hard-pressed to define it, I suppose

The fundamental effect of an object on the Universe, resulting from the object's existence, causing it to affect other objects.
A fundamental phenomenon of nature, which, when applied to an object, causes the object to move in a well-defined direction, alters the object's path of motion, or overcomes another force applied in an opposing direction.
will have to do, even though the latter will make holists wince. Physicists can later quantify things an point out that such a "tendency to move" is really an acceleration and is proportional to the object's inertia or mass. Then they can get into relativity. We've at least passed the buck onto the poor slob who has to define motion.

Our macroscopic notions of "force" are the effects we see from countless interactions between pairs of particles (wavefunctions) at the quantum-mechanical level: potential energy builds up between two particles, until there is enough energy to spontaneously create another pair, which travels from one to the other, changing the trajectories of both particles:

   \             /
    \           /
     \         /
     /         \
    /           \
   /             \

All of our day-to-day experience comes from innumerable little catastrophes of this sort, almost all of which are photons scattering pairs of electrons in our bodies' atoms.

The Standard Model of physics currently defines three "interactions" which cause forces to be exerted on objects:

Physics describes dozens of other "forces" which are really special cases of the above three interactions: things sich as centripetal forces, Van der Waals forces. Other, more complex effects such as the "Coriolis effect" or the "centrifugal effects" are erroneously termed as forces.

A notable exception is dark energy (sometimes called antigravity), not observed in our local environment, but which must exist due to cosmological inflation and the fact that the expansion of the universe is accelerating. This may be a special case of gravity, arising from Albert Einstein's cosmological constant.

It should be apparent from the second definition given above that force is a vector concept: The notion of a "direction" wouldn't be necessary if forces couldn't cancel each other out. At each point of the universe, then, each of the fundamental forces are is added up from all other points of the Universe and a final "result" vector is produced, the direction the object will accelerate. This gave rise to the field concepts of physics which are still useful in our macroscopic world. The field concept brings up notions of action at a distance, which in our macroscopic world, things like gravity and magnetism appear to be.

Quantum physics describes forces as derived from vector bosons emitted by particles and intercepted by other particles, changing their relative energy balances. The field concept lives even here, however: It is plausible to construct as a model of physics, consisting of only forces, with particles existing only as specific wavefunctions in these Universe spanning force fields.

There are at least three different units of force:

  • The newton (kg*m/s2) an MKS SI unit.
  • The dyne (g*cm/s2), a cgs SI unit.
  • The pound from the English system of measurement used primarily in the United States. This unit was arbitrarily derived from the relative weight of objects at the Earth's surface.