Standard ZDoom units

distance – mapunits

time – tics (or seconds in non-gameplay contexts)

angle – turns counterclockwise

Derived units

velocity – mapunits per tic

acceleration – mapunits per tic^2

Gravity is defined by free fall acceleration, and is measured using the same units as normal acceleration.

Non-standard units

These units should be converted to standard units before performing any operations with them.

Space

meters, feet, and other nonsense – Doom doesn't have an accurate scale

Time

seconds (35:1)

octics (4.375:1)

Angles

degrees (360:1)

radians (tau:1)

byte angles (256:1)

sv_gravity units (800:1)