The Merriam-Webster dictionary now includes a secondary definition of marriage to recognise same-sex relationships - in fact, years before gay couples could legally tie the knot anywhere in the United States.

Although the update was made in 2003, the change only gained widespread notice after the conservative World Net Daily news site published an article this week criticising Merriam-Webster for the addition.

The definition now includes the phrase: "The state of being united to a person of the same sex in a relationship like that of a traditional marriage."

Merriam-Webster, widely regarded as the leading dictionary in the US, issued a statement on Wednesday explaining that the edited entry merely reflected the frequency with which the term "same-sex marriage" had popped up in print and become part of the general lexicon.

"In recent years, this sense of 'marriage' has appeared frequently and consistently through a broad spectrum of carefully edited publications, and is often used in phrases such as 'same-sex marriage' and 'gay marriage' by proponents and opponents alike," the statement read. "Its inclusion was a simple matter of providing dictionary users with accurate information about all of the word's current uses."

"What we are finding odd is that this is neither news nor unusual," spokesman Arthur Bicknell said. "In fact, we were kind of late to the party. We were one of the last ones among the major dictionary publishers to do this."

Other American dictionaries such as the American Heritage Dictionary in 2000 added a forth clause that states: "A union between two persons having the customary but usually not the legal force of marriage: a same-sex marriage."

The British Oxford English Dictionary which defines marriage as a "condition of being a husband or wife; the relation between persons married to each other; matrimony" in 2001 added, "the term is now sometimes used with reference to long-term relationships between partners of the same sex."

In the US, Connecticut and Massachusetts are the only states where same-sex marriage is legal; the unions are however not recognised nationally. The Netherlands, Belgium, Spain, Canada, South Africa and Norway recognises same-sex marriage.