A toddler is a small child at the stage just after baby — old enough to be walking, but still wobbly at it. Roughly one to three years old, though nobody checks a birthday before using the word; what makes a toddler is the walking, the falling over, the grabbing at everything, and usually a few words rather than sentences. The name is literal: to toddle is to walk with short unsteady steps, so a toddler is 'one who toddles'. English needs this word because baby and child are both wrong here — a baby cannot walk, and child sounds too old. You will see it constantly on parenting websites, in clothing sizes, and in shop signs (toddler shoes, toddler class). It is a warm, ordinary word, and adults borrow it as a joke for grown-ups who are behaving badly — 'don't be such a toddler about it' means stop having a tantrum. There is also a common phrase, 'the terrible twos', for the age when toddlers say no to everything.