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.).
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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.