bloodhound

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. One of a breed of hounds with a smooth coat, drooping ears, sagging jowls, and a keen sense of smell.
2. n. Informal A relentless pursuer.

Century Dictionary and Cyclopedia

1. n. A variety of dog with long, smooth, and pendulous ears, remarkable for the acuteness of its smell, and employed to recover game or prey which has escaped, tracing a wounded animal by the blood it has spilled (whence its name), or by any other effluvium or halitus left on a trail which it follows by scent. There are several varieties of this animal, as the English, the Cuban, and the African bloodhound. Bloodhounds are often trained not only to the pursuit of game, but also of man, as of fugitive criminals; in the United States they were formerly employed in hunting fugitive slaves.
2. n. Figuratively, a man who hunts for blood; a relentless persecutor.

Wiktionary

1. n. A large scenthound famed for its ability to follow a scent many days old, over vast distances. This dog is often used as a police dog to track missing people, fleeing suspects, or escaped prisoners.
2. n. figuratively A detective or other person skilled at finding people or clues.

GNU Webster's 1913

1. n. A breed of large and powerful dogs, with long, smooth, and pendulous ears, and remarkable for acuteness of smell. It is employed to recover game or prey which has escaped wounded from a hunter, and for tracking criminals. Formerly it was used for pursuing runaway slaves. Other varieties of dog are often used for the same purpose and go by the same name. The Cuban bloodhound is said to be a variety of the mastiff.

WordNet 3.0

1. n. a breed of large powerful hound of European origin having very acute smell and used in tracking