Hart is an archaic word for "stag" (from Old English heorot, "deer" – compare with modern Dutch hert, German Hirsch and Swedish/Norwegian/Danish hjort, also "deer").
Specifically, "hart" was used in medieval times to describe a red deer stag more than five years old.
In medieval hunting terms, a stag in its first year was called a "calf" or "calfe", in its second a "brocket", in its third a "spayed", "spade", or "spayard", in its fourth a "staggerd" or "staggard", and in its fifth a "stag", or a "great stag". To be a "hart" was its fully mature state. A lord would want to hunt not just any deer, but a mature stag in good condition, partly for the extra meat and fat it would carry, but also for prestige. Hence a hart could be designated "a hart of grease", (a fat stag), "a hart of ten", (a stag with ten points on its antlers) or "a royal hart" (a stag which had been hunted by a royal personage). A stag which was old enough to be hunted was called a "warrantable" stag.
The hart was a "beast of venery" representing the most prestigious form of hunting, as distinct from lesser "beasts of the chase", and "beasts of warren", the last of which were regarded virtually as vermin. The membership of these different classes varies somewhat, according to which period, and which writer, is being considered, but the red deer is always in the first class, the fox hardly being regarded at all. Like the fallow deer buck and the wild boar, the hart was normally sought out or "harboured" by a "limer", or bloodhound hunting on a leash, which would track it from its droppings or footprints to where it was browsing. The huntsman would then report back to his lord and the hunting party would come bringing a pack of raches. These scent hounds would "unharbour" the hart and chase it on its hot scent until it was brought to bay.
The word hart is not now widely used, but Shakespeare makes several references (for example in Twelfth Night), punning on the homophones "hart" and "heart". The word is also used several times in The Hobbit by J.R.R. Tolkien, when Bilbo Baggins and company pass through Mirkwood Forest. It is also alluded to in the Joss Whedon series Angel; the Senior Partners of law firm Wolfram & Hart are represented, respectively by the Wolf, the Ram and the Hart. "The White Hart", a personal emblem of Richard II, and "The Red Hart" remain common English pub names. "White Hart Lane" is the name of the stadium of Tottenham Hotspur F.C. of the English Premier League. The county Hertfordshire (along with Hertford, its county town, Hartford, its twin town in Connecticut and the village of Hartford, near Northwich, in Cheshire) is thought to be named after a place where deer forded a watercourse. There is also the district of Hart in Hampshire and the villages of Hartfield at the edge of Ashdown Forest in East Sussex and Hart Common on the outskirts of Westhoughton, in Greater Manchester, and the town of Hartlepool in County Durham. Whinfell Forest once contained a landmark tree called Harthorn. The surnames Hart and Hartley ("wood of the hart") also derive from the animal.
It appears in the first line of Psalm 42 in the King James Authorized Version (1604-1611) of the Psalms: "As the hart panteth after the water brooks, so panteth my soul after thee, O God." Tate and Brady's (1696) metrical psalms, among others, also use this figure: "As pants the hart for cooling streams" for its Common Meter (CM) rendering of the Psalm 42 text.