Unusual words and difficult words, including idioms and jargon, need to have definitions, so users know what they mean. An unusual word is one that’s not used in everyday conversation, 

The first time an unusual word or phrase is used we should write an inline definition.

If the word or phrase has multiple meanings in the same page, then we should write an inline definition each time there is a new meaning.

‍‍This is an accessibility guideline, meeting Success Criterion 3.1.3 Unusual Words, level AAA.