A principle or aid, adapted to solving problems of some given type. Colloquially, a heuristic is a "rule of thumb". A heuristic should not be considered a hard and fast rule.
Due to the large size of the Go board, players often use heuristics, either consciously or unconsciously, to decide where they should play. It is common to use heuristics when recognizing an eye, even though there are exceptions to such heuristics. At the same time, many proverbs serve as heuristics, and players are advised not to follow proverbs blindly.
heuristic (1): adjective: serving to guide, discover, or reveal; specifically: valuable for empirical research but unproved or incapable of proof. (German heuristisch, from New Latin heuristicus, from Greek heuriskein to discover; akin to Old Irish fuar and Greek eureka! I have found)
heuristic (2): noun 1: the study or practice of heuristic procedure; 2: heuristic argument
The common usage in computing, especially artificial intelligence, is as a noun meaning a technique to be tried by a program when a direct definitive algorithm or exact solution is not available.