If you are writing a manual for electrical engineers, you do not need to explain what Ohm's Law is. Actually, if you do, you may insult and alienate your highly educated readers.

If, however, you are writing a children's book or a general purpose newspaper article, you have to make sure to define the “Ohm's Law.”

In a peer-reviewed article on a neurological breakthrough, you do not need to define what an “axon” is. If you do, you'll insult and alienate your audience and it probably won't survive editing anyways.

But if you are writing the same article for a magazine read by seniors and retired people, it'd help to provide a definition.

(An excerpt from my book 101 Ways to Power-Up Your Writing )