Real Dictionary

agar in American English
a gelatinous product made from seaweed and used as a base for bacterial cultures, as a laxative, in jellied and preserved foods, in electrophoresis, etc.
noun
noun: a gelatinlike product of certain seaweeds, used for solidifying certain culture media, as a thickening agent for ice cream and other foods, as a substitute for gelatin, in adhesives, as an emulsifier, etc

agar in British English
noun: a complex gelatinous carbohydrate obtained from seaweeds, esp those of the genus Gelidium, used as a culture medium for bacteria, a laxative, in food such as ice cream as a thickening agent (E406), etc