What is the ruling on swallowing phlegm while fasting, with a definition of phlegm?

Phlegm is one of the body's humors. Swallowing it after it reaches the mouth is a matter of scholarly disagreement. Sheikh Al-Uthaymeen favored that swallowing it does not invalidate the fast, because the default is that an act of worship is not invalidated. The important thing is that a person should not draw the phlegm from his throat; rather, he should expel it once it reaches the mouth. However, to be on the safe side, it is better to avoid it, as a way of exiting the disagreement.