The Sanskrit word guna (guṇa) has the basic meaning of "string" or "a single thread or strand of a cord or twine". In more abstract uses, it may mean "a subdivision, species, kind", and generally "quality".
In Classical literature (e.g. Mahabharata, Bhagavata Purana), a Guna is an attribute of the 5 elements (each with an associated organ):
1. ether has sound (shabda) for its Guna (and the ear for its organ).
2. the air has tangibility and sound for its Gunas (and the skin for its organ).
3. fire has shape or colour, tangibility, and sound for its Gunas (and the eye for its organ).
4. water has flavour, shape, tangibility, and sound for its Gunas (and the tongue for its organ).
5. earth has all preceding Gunas, plus its own peculiar Guna of smell (and the nose for its organ).
In Samkhya philosophy
In Samkhya philosophy a Guna is one of three "tendencies" or "mental states": tamas, sattva, and rajas.
These categories have become a common means of categorizing behavior and natural phenomena in Hindu philosophy, and also in Ayurvedic medicine, as a system to assess conditions and diets.
Sattva (originally "being, existence, entity") has been translated to mean balance, order, or purity. This typically implies that a person with this quality has a positive or even orderly state of mind. Such a persion is psychologically kind, calm, alert and thoughtful. Compare also the bodhisattvas in Buddhism.
Rajas (originally "atmosphere, air, firmament") has been translated to mean overactivity or turmoil: "too active". A person with this mental state has a mind that is ever active, in turmoil, or in a chaotic state. That person is constantly seeking diversions and essentially has difficulty focusing their attention for long durations of time. (Rajas should not be confused with a raja.)
Tamas (originally "darkness", "obscurity") has been translated to mean "too inactive", negative, lethargic, dull, or slow. Usually it is associated with darkness or ignorance. A tamas quality also can imply that a person has a self-destructive or entropic state of mind. That person is constantly pursuing destructive activities.
In Nyaya philosophy
In Nyaya philosophy, twenty-four Gunas are enumerated as properties or characteristics of all created things.
1. rūpa, shape, colour;
2. rasa, savour;
3. gandha, odour;
4. sparśa, tangibility;
5. saṃkhyā, number;
6. parimāṇa, dimension;
7. pṛthaktva, severalty;
8. saṃyoga, conjunction;
9. vibhāga, disjunction;
10. paratva, remoteness;
11. aparatva, proximity;
12. gurutva, weight;
13. dravatva, fluidity;
14. sneha, viscidity;
15. shabda, sound;
16. buddhi or jñāna, understanding or knowledge;
17. sukha, pleasure;
18. duḥkha, pain;
19. icchā, desire;
20. dveṣa, aversion;
21. prayatna, effort;
22. dharma, merit or virtue;
23. adharma, demerit;
24. saṃskāra, the self-reproductive quality;
In grammar
In Sanskrit grammar, guṇa is a technical term referring to the vowels a, e, o, i.e. the full grade ablaut stages (see Ashtadhyayi).
References
The Ayurveda Encyclopedia by Swami Sada Shiva Tirtha
