Physical quantities: the nouns physics is written in

Tell a shopkeeper you want “a lot” of sugar and he will just stare at you. Tell him 2 kg, and the deal is done. Physics has exactly the same problem with the whole universe: vague words are useless, so everything gets a number and a unit.

A physical quantity is exactly this pair: a number plus a unit. Length, mass, time, velocity, force, density and temperature all fit the same mould, even though some are simple and some are built out of others.

So physics splits its quantities into two families. Base quantities stand on their own. Derived quantities are always someone else's combination — velocity is just distance divided by time wearing a different name.