This weekend, my daughter chuckled over my using entspannen in an English sentence. But I love the word! The verb spannen means to tighten, stress, stretch, or otherwise put tension on something. The prefix ent- suggests something at a distance. So entspannen means to remove the source of tension or stress–to relieve pressure or to ease up on a spring. This carries over to emotional stress. Sich entspannen means to relax. So, for example, Ich entspanne mich beim reiten–I relax by hoseback riding.
I really like that this verbe is reflexive (or, in other words, takes the sich). Because that expresses the truth. We can’t just relax. We have to relax ourselves. We take the stress off ourselves in proactive, healthy ways.