Although it’s quite common for people to use the word relaxen (although this is typically greeted with a frown of disapproval by older people, similar to chillen), the proper German word for “to relax” is entspannen, used with the reflexive sich/mich/dich, etc.

Additionally, the (also reflexive) erholen is used to express “to relax” in the sense of recovering (perhaps after a rough week at the office).

The nouns are Entspannung (f.) and Erholung (f.).

Examples

Ich entspanne mich am Wochenende. I am relaxing on the weekend.

Nach dieser Woche müssen wir uns mal so richtig entspannen. After this week we’ve really got to relax.

Ich erhole mich beim Pool. I’m relaxing/recuperating at the pool.

Er erholt sich von seinem Arbeitsstress. He’s recovering from his work stress.