The term is most commonly used to describe any male person, who is in a romantic relationship with any other person of either gender.
Such non-marital relationships are also sometimes described as a significant other or partner, especially if the individuals are cohabitating. The differences between all these terms are subjective and their usage is ultimately be determined by personal preference.
The word itself is relatively new — its first usage in print known to the Oxford English Dictionary is in George W. E. Russell’s Collections and recollections, by one who has kept a diary, in 1909.
In the past it had implications of an illicit relationship (as sexual and romantic relationships outside marriage were generally frowned upon). It is now a generally accepted term, however, no longer having negative connotations. An earlier usage in print, dating from July 1889, is discussed in Neil Bartlett, Who Was That Man? A Present for Mr Oscar Wilde. On pages 109-110, Bartlett quotes from an issue of The Artist and Journal of Home Culture, which refers to Alectryon as “a boyfriend of Mars.”
Dating is the activity of looking for a suitable partner for an intimate relationship. The word refers to two daters agreeing on a time and date when they can meet and engage in some activity, thereby assessing their suitability for one another. Typically a person may date many different partners during the same time period in order to have the best chance of finding their most suitable available mate. As in the marketplace, liquidity is essential for all participants to achieve the best outcome available to them.