Ockham’s Razor is the heuristic sometimes known as the lex parsimoniae: the Law of Parsimony. As he actually proposed it:
Numquam ponenda est pluralitas sine necessitate: Do not posit pluralities beyond necessity.
Ockham’s Razor as it is usually rendered:
Entia non sunt multiplicanda praeter necessitatem: Do not multiply entities beyond necessity.
The entities of a theory are its terms. They are not actual entities, but formal only. So the Razor is often rendered:
Do not multiply terms beyond necessity.
This makes it easy to compare theories and see which one is more parsimonious – especially if they are mathematically formalized. F = ma, for example, clearly invokes three terms, that terminate on three sorts of properties of things. The basic idea of course is that as between two theories that adequately explain some phenomenon, the simpler is more likely to be more accurate. But why?