From the Latin cadere 'to fall', the cadent houses are those which ‘fall away’ from the angles by diurnal revolution: the 3rd, 6th, 9th and 12th houses. Considered weak areas for planetary expression.
Planets are also said to be weak and badly placed when they are 'cadent from their own house (sign)'. William Lilly mentions this is regard to fertility matters (p.227) where he explains the principle as follows:
[Querent is likely to be pregnant if the Moon is ....] applying to the Lord of the Ascendant or Lord of the 5th in the 1st or 10th house, and he [5th-ruler] is not Cadent from his own House or exaltation; where you must understand this general rule concerning a Planet his being Cadent from his own House, is this, viz. if Mars be in Aries, it being his own House, let him then be in any of the twelve Houses, he shall be said to be Angular as to his being in Aries: if Mars be in Taurus he is Succeeding or in a Succedent House in that way: if Mars be in Gemini he is then Cadent as from his owne House; and so forth in the rest: for ever a Planet is Angular in any of his own Houses.