a. Having a temper not easily excited; free from passion; unshakably calm and collected. -- Cool"-head`ed*ness, n. Syn. -- cool, imperturbable, unflappable. [1913 Webster + WordNet 1.5]
(adj) marked by calm self-control (especially in trying circumstances); unemotional, Syn. coolheaded, nerveless, Example:play it cool; keep cool; stayed coolheaded in the crisis; the most nerveless winner in the history of the tournament