Concise Pali-English Dictionary by A.P. Buddhadatta Mahathera
upakkilesa:[m.] impurity; defilement; anything that spoils or obstructs.
Buddhist Dictionary by NYANATILOKA MAHATHERA
upakkilesa:'impurities',corruptions,imperfections (a frequent rendering by 'defilements' is better reserved for kilesa,q.v.).
A list of 16 moral 'impurities of the mind' (cittassa upakkilesa ) is mentioned and explained in M.7 & 8 (WHEEI.61/62):1.covetousness and unrighteous greed (abhijjhā-visamalobha),2.ill will (byāpāda),3.anger (kodha),4.hostility (upanāha),5.denigration (makkha),6.domineering (palāsa),7.envy (issā),8.stinginess (macchariya),9.hypocrisy (māyā),10.fraud (sāṭheyya),11.obstinacy (thambha),12.presumption (sārambha),13.conceit (māna),14.arrogance (atimāna),15.vanity (mada),16.negligence (pamāda).
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,q.v.),mentioned in Pts.M.,Ānāpāna-kathā (tr.in Mindfulness of Breathing,by Ñāṇamoli Thera (p.60; BPS).
(c) 10 'imperfections of insight' (-meditation,vipassanūpakkilesa); s.visuddhi V.