In logic:
A well-formed formula (wff) is a string of symbols from the alphabet of a (formal) language, that conforms to the grammar of the language.

For example: Let's look at a language L. L contains the symbols A and B only, with the following grammar rules:

1. A and B are wffs.
2. If C and D are wffs then (C v D) is a wff.

Admittedly not a very exciting or useful language. But it serves us well enough for this example. So, in L:

• A is a wff
• A v B is not
• (A v (A v A)) is a wff.
• (A v B v A) is not