Real Dictionary

gluten
uncountable noun: Gluten is a substance found in cereal grains such as wheat.

gluten in American English
a gray, sticky, nutritious mixture of proteins, including gliadin, found in wheat and other grain: it gives dough its tough, elastic quality
noun: the tough, viscid, nitrogenous substance remaining when the flour of wheat or other grain is washed to remove the starch

gluten in British English
noun: a protein consisting of a mixture of glutelin and gliadin, present in cereal grains, esp wheat. A gluten-free diet is necessary in cases of coeliac disease