General:
* Расслабиться (rasslabit'sya) - This is the most common and versatile way to say "relax". It can be used in almost any situation.
More specific:
* Отдохнуть (otdokhnut') - This means "to rest" or "to take a break" and is often used for physical relaxation.
* Успокоиться (uspokoitsya) - This means "to calm down" and is used when you need to relax after being stressed or anxious.
* Позволить себе расслабиться (pozvolit' sebe rasslabit'sya) - This means "to allow oneself to relax" and implies a conscious decision to relax.
Informal:
* Отдыхай (otdykhaj) - This is the informal imperative form of "отдохнуть" and is a more casual way to tell someone to relax.
* Спокойно (spokojno) - This means "calmly" and is often used as a way of telling someone to relax or to take things easy.
The best way to choose the right word depends on the specific situation and the level of formality you want to use.