pessimum
pessy-mum
noun
The least favourable circumstances in which an organism can survive
Anyone who has ever tried to keep an orchid alive will know that some species are remarkably choosy about the conditions in which they live. On the other hand, some species are remarkably hardy, and can thrive wherever you plonk them. Unenlightened types might call such organisms ‘weeds’, but I prefer the term ‘pioneers’.
In the realm of biology the words ‘optimum’ and ‘pessimum’ refer, respectively, to the most and least favourable conditions in which an organism can survive. Go beyond the pessimum in whatever metric you choose (too hot, too cold, too much water, not enough sunlight, soil too acidic, not enough coffee shops nearby etc.) and the organism will die.
Eagle-eyed types will have spotted that the ‘optimum’ / ‘pessimum’ pair bears a striking similarity to another, perhaps better-known pair of words: ‘optimist’ and ‘pessimist’, and will be gratified to know that all these words share the same roots: ‘optimus’ and ‘pessimus’, Latin for ‘best’ and ‘worst’ respectively.