A line of sunlight — one narrow shaft of it, travelling from the sun to whatever it lands on.

The key idea is that you can see its shape. Ordinary daylight fills a room and has no edges. A sunbeam has edges: it comes through a gap in the curtains, a hole in the clouds, a crack in a wooden wall, a gap in the trees of a forest, and lies across the floor as a bright stripe. Dust floating in the air is what usually makes it visible — the tiny pieces catch the light so the beam looks almost solid, like something you could touch. That is why people say a cat is 'lying in a sunbeam' on the carpet, and why the word feels warm and homely.

The feeling of the word is gentle and slightly old-fashioned, closer to poems, children's books, and hymns than to science writing, which would say 'ray' or 'solar radiation'. Because of that warmth, English also uses it for people: a cheerful child, or someone who makes a room feel better, may be called 'a little sunbeam'. That use is affectionate and mostly said to or about children — said to an adult, it sounds either sweet or slightly sarcastic, depending on the face that says it. There is a well-known sarcastic version too: 'aren't you a ray of sunshine' means the opposite.

'Sunbeam' and 'ray of sunlight' mean almost the same thing, and either will do in nearly every sentence.