Upakkilesa: '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 WHEEI. 61/62: 1. covetousness and unrighteous greed abhijjhā-visamalobha 2. ill will vyāpāda. anger kodha 4. hostility upanāha. denigration makkha 6. domineering palāsa 7. envy issā 8. stinginess macchariya 9. hypocrisy māyā 0. fraud sātheyya 11. obstinacy thambha 12. presumption sārambha 13. conceit māna, 4. arrogance atimāna 5. vanity mada 16. negligence pamāda.
There are 3 groups of upakkilesa pertaining to meditation:
- 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.
- 18 imperfections in the practice of awareness or mindfulness of breathing ānāpāna-sati, mentioned in Pts.M., Ānāpāna-kathā tr. in Mindfulness of Breathing, by Ñānamoli Thera p. 60; BPS.
- 10 'imperfections of insight' -meditation, vipassanūpakkilesa see: visuddhi V.
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952.