An optimum is the best condition or highest extent of something. He didn't mean to create a science experiment, but the unwashed coffee cup under his desk became the optimum dark, wet environment for growing lots of mold.
Optimum came into English in the 19th century from the Latin optimus, "best." You can use optimum to describe the best conditions for meeting a goal or achieving something, and it often refers to conditions in scientific tests. Runners determine their optimum weight before running a marathon, and astronauts train in the optimum environments for recreating what they'll experience in space. Optimum conditions for a great barbecue might include sunshine and blue skies.