'impurities', corruptions, imperfections (a frequent rendering by 'defilements' is better reserved for kilesa).
A list of 16 moral 'impurities of the mind' (cittassa upakkilesa) is mentioned and explained in M.7 & 8 (WHEEL. 61/62):
There are 3 groups of upakkilesa pertaining to meditation:
(a) 9 mental imperfections occurring in 'one devoted to higher mental training' (adhicitta); 3 coarse ones - evil conduct in deeds, words and thoughts; 3 medium - thoughts of sensual desire, ill will and cruelty; 3 subtle - thoughts about one's relatives, one's country and one's reputation (A.III.100).
(b) 18 imperfections in the practice of mindfulness of breathing (ānāpāna-sati), mentioned in Pts.M., ānāpāna-kathā (tr. in Mindfulness of Breathing, by Ñānamoli Thera (p. 60; BPS).
(c) 10 'imperfections of insight' (-meditation, vipassanūpakkilesa); s. visuddhi V.