Full Idea
We write 'not-P' as ¬P. This is called the 'negation' of P. The 'double negation' of P (not not-P) would be written as ¬¬P.
Gist of Idea
We write the 'negation' of P (not-P) as ¬
Source
E.J. Lemmon (Beginning Logic [1965], 1.2)
Book Reference
Lemmon,E.J.: 'Beginning Logic' [Nelson 1979], p.7
A Reaction
Lemmons use of -P is no longer in use for 'not'. A tilde sign (squiggle) is also used for 'not', but some interpreters give that a subtly different meaning (involving vagueness). The sign ¬ is sometimes called 'hook' or 'corner'.