Phlegm is the humour of this temperament, cold and moist, and therefore paired with water among the elements, with winter among the seasons, and with old age among the ages of life. Its traditional seat is the brain, and the visible evidence for it was the mucus of colds and catarrhs, which increases in winter and which the scheme took as the humour made apparent.
The word has an odd history. The Greek phlegma comes from a root meaning to burn and originally denoted inflammation, which is the opposite of what it came to mean; the humoral system fixed it as the cold wet humour and the older sense fell away. That reversal is a useful reminder that the four names were not coined for the system. They were drawn from existing medical vocabulary and then given fixed places in a structure that vocabulary had not been built to serve, which is exactly what happened to the angelic ranks in a quite different field.
In treatment, phlegm called for heating and drying: warm and pungent preparations, exercise, and the avoidance of cold damp air and cold damp food. Because the humour was held to accumulate with age and with winter, an old person in a cold country was understood to be fighting a standing excess, and a great deal of the regimen literature is addressed to precisely that situation.