What counts as a Term or a URL?

One Term = 1 search term tracked for 1 URL on 1 search engine.

Every URL/Term/Search-Engine combination counts as 1.

Every Local combination (i.e: different city) or a Mobile combination (different mobile device) counts as 1.

The terms limit of a plan is Total. 
For example, The Starter plan's limit is 500. With this capacity you can have 10 URLs X 50 Terms each (or 500 URLs X 1 Term each).

Unlimited URLs means that there is no terms restriction per URL,
So the actual URL number is unlimited up to the terms capacity in case of tracking 1 term per 1 URL.