A euphemism is the substitution of an agreeable or less offensive expression in place of one that may offend or suggest something unpleasant to the listener; or in the case of doublespeak, to make it less troublesome for the speaker. It also may be a substitution of a description of something or someone rather than the name, to avoid revealing secret, holy, or sacred names to the uninitiated, or to obscure the identity of the subject of a conversation from potential eavesdroppers. Some euphemisms are intended to be funny.
Reference:
http://en.wikipedia.org/wiki/Euphemism
Reference:
http://en.wikipedia.org/wiki/Euphemism