I thought I knew what work means, but my physics teacher defines it differently. What's up with that?

The concept of work in physics is much more narrowly defined than the common use of the word. Work is done on an object when an applied force moves it through a distance. In our everyday language, work is related to expenditure of muscular (or mental) effort, but this is not the case in the language of physics.

A person that holds a heavy object does no physical work, because the force is not moving the object through a distance. Work, according to the physics definition, is being accomplished while the heavy object is being lifted but not while the object is stationary.

Mathematically, work is W = F × x, where F is the applied force and x is the distance moved, that is, displacement. Work is scalar. The SI unit for work is the joule (J), which is newton-meter or kg m/s2.