Both dictionary.com and m-w.com (Marriam-Webster dictionary) agree with your definition, with the addition of "...without being legaly married to him" (thus agreeing with Casiopea).
Secondary definitions from these two dictionaries include:
-"In certain societies, such as imperial China, a woman contracted to a man as a secondary wife, often having few legal rights and low social status."
-"One having a recognized social status in a household below that of a wife"
-"A wife of inferior condition; a lawful wife, but not united to the man by the usual ceremonies, and of inferior condition."
Dictionary.com definition Merriam Webster definition
Hope this helps.