Real Dictionary

relax
verb: If you relax a rule or your control over something, or if it relaxes, it becomes less firm or strong.
verb: If you relax or if something relaxes you, you feel more calm and less worried or tense.
verb: If you relax your grip or hold on something, you hold it less tightly than before.
verb: When a part of your body relaxes, or when you relax it, it becomes less stiff or firm.

relax in American English
to make looser, or less firm or tense
intransitive verb: to become less tense, rigid, or firm
transitive verb: to make less tense, rigid, or firm; make lax

relax in British English
verb: to make (muscles, a grip, etc) less tense or rigid or (of muscles, a grip, etc) to become looser or less rigid