n. One of several large breeds of dog (such as bulldogs and Saint Bernards), often used as guard dogs
the GNU version of the Collaborative International Dictionary of English
n. A breed of large dogs noted for strength and courage. There are various strains, differing in form and color, and characteristic of different countries.
The Century Dictionary and Cyclopedia
n. A variety of dog of considerable antiquity.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. an old breed of powerful deep-chested smooth-coated dog used chiefly as a watchdog and guard dog
Word Usage
"The mastiff is a large, grave, sullen-looking dog, with a wide chest, noble head, long switch tail, bright eyes, and a loud, deep voice."