Longman Dictionary of Contemporary English
The Collaborative International Dictionary
double standard \dou"ble stan"dard\, n. a standard or set of principles governing conduct, which is applied more stringently or differently to one group of people than to another; -- used especially of standards of sexual behavior that condemn behavior on the part of women that is condoned or not condemned when exhibited by men.
Wiktionary
n. 1 The situation of two or more groups, one of whom is tacitly excused from following a standard generally regarded as applying to all groups. 2 A double standard of monetary values, i.e. a gold standard and a silver standard, both of which are legal tender.
WordNet
n. an ethical or moral code that applies more strictly to one group than to another
Wikipedia
A double standard is the application of different sets of principles for similar situations.
A double standard may take the form of an instance in which certain concepts (often, for example, a word, phrase, social norm, or rule) are perceived as acceptable to be applied by one group of people, but are considered unacceptable— taboo—when applied by another group. A double standard can therefore be described as a biased or morally unfair application of the principle that all are equal in their freedoms. Such double standards are seen as unjustified because they violate a basic maxim of modern legal jurisprudence: that all parties should stand equal before the law. Double standards also violate the principle of justice known as impartiality, which is based on the assumption that the same standards should be applied to all people, without regard to subjective bias or favoritism based on social class, rank, ethnicity, gender, religion, sexual orientation, age, or other distinctions. A double standard violates this principle by holding different people accountable according to different standards.