What is debilitation?

Some articles on debilitation:

Rahu - Astrology
... Rahu's rulership, exaltation, debilitation, and mulatriko?a are topics of controversy, as many views abound as to the sign that Rahu rules ... assign Rahu different signs regarding rulership, exaltation, and debilitation ... In establishing the rulership, exaltation, and debilitation signs of Rahu, its relationship with other planets may be considered Rahu is friends with Ketu, Saturn, Mercury and Venus ...