(anatomy) back (of a person) / (in the plural) backs (of persons) / (swimming) backstroke

(literally) back to back

a move in square dancing in which two dancers face each other, then step forward and left until they have right shoulders adjacent, then move to a position where they are back to back, then move to have left shoulders adjacent, then return to facing each other

back-to-back

back-to-back

backstroke

(idiomatic) speed bump; sleeping policeman

backsaw (handsaw that has a reinforcement along its back edge.)

rucksack, backpack (bag worn on a person's back)

(figuratively, transitive with à) to turn one's back on, to turn away from