Optimum means the best or most right for a particular purpose or situation — not “best in the world”, but “best for this”. You can say the optimum (a noun: the best point or amount) or optimum temperature, optimum size (in front of another noun). Scientists, doctors, and planners like this word because the best amount is often not the biggest amount: a plant dies if the room is too hot or too cold. Though actually, in everyday talk some people just use it as a fancy “best” or “ideal”. Optimal is a close sister word, used mostly as an adjective; optimum does both jobs.