【Speed】 ,  【velocity】 ,  【momentum】 ,  【impetus】 ,  【pace】 ,  【headway】  are comparable but not all mutually synonymous terms that basically apply to motion through space.

【Speed】  (see also HASTE ) denotes rate of motion, a value computable by dividing the distance covered by the time taken.

【Velocity】  denotes the 【speed】 of something that is directed along a given path. Ordinarily  【velocity】  suggests rate of motion in a straight line (linear 【velocity】  ) or in an arc or circle (angular 【velocity】 ).

【Momentum】  in general or casual use may take the place of  【speed】  or  【velocity】 , but in technical use it denotes not rate of motion but quantity of motion, a value determinable by multiplying the mass of a moving body by its 【velocity】 .

【Impetus】  (see also STIMULUS ) is a popular rather than technical synonym of  【momentum】 . In practice it is a closer synonym of impulse in the sense of the effect of an impelling force, for it regularly carries an implication of a rushing upon or an onset, with the result that it usually suggests great 【momentum】 or implies a powerful driving force as the cause of such 【momentum】 .

【Pace】 belongs here not as a technical term but as a term in general use to denote the 【speed】 of or as if of one (as a person or horse) going afoot.

【Pace】 often finds extended use in describing such things as activities, progress, or rate of production.

【Headway】 basically applies to motion forward and in this sense is used chiefly in reference to ships. However it frequently denotes rate of movement ahead and then is used in reference not only to ships but to whatever is capable of advancing or making progress.