Sun bathing is when you lie outside on a summer day and soak in the sunshine. Many people enjoy sun bathing when they go to the beach.
You can spell sun bathing as a single word or as two separate words — and you can also call it sun tanning, although sun bathing can mean simply enjoying the warmth of the sun on your skin, rather than going for a dark tan. The term became popular in the 1940's, when people began to seek the sun — and resulting tanned skin — for health and fashion reasons. The term sunbather seems to have first appeared in 1941, with sun bathing following closely behind.