If you straighten something, you make it tidy or put it in its proper position. She sipped her coffee and straightened a picture on the wall. tidying, straightening cushions and organising magazines
If you straighten something, or it straightens, it becomes straight. Straighten both legs until they are fully extended The road straightened and we were on a plateau. Straighten out means the same as straighten. No one would dream of straightening out the knobbly spire at Empingham Church The road twisted its way up the mountain then straightened out for the last two hundred yards
get up from a sitting or slouching position; "The students straightened when the teacher entered"
make straight or straighter; "Straighten this post"; "straighten hair" get up from a sitting or slouching position; "The students straightened when the teacher entered" straighten up or out; make straight