In recent years, a number of journalism style guides have allowed the gender-neutral pronouns. The Associated Press in 2017 announced it would permit journalists to use the singular "they" in limited cases, and the Washington Post has formally recognised the new pronouns since 2015.
But in a blog post written before Wednesday's announcement, Merriam-Webster noted that "they" has been used as a singular pronoun since the late 1300s, "and that regardless of what detractors say, nearly everyone uses the singular 'they' in casual conversation and often in formal writing".
"There have always been people who didn't conform to an expected gender expression, or who seemed to be neither male nor female. But we've struggled to find the right language to describe these people — and in particular, the right pronouns," the blog post stated.
The singular 'they', it said, is "not quite as newfangled as it seems: we have evidence in our files of the nonbinary "they" dating back to 1950, and it's likely that there are earlier uses of the nonbinary pronoun 'they' out there".
Many English speakers consider dictionaries to be "constitutional" authorities, Baron said.
"They want to have some kind of stability in language that they can point to and say 'here's the rules'."
In reality, Baron said, dictionaries are not intended to set rules on how people should behave. "They're a general indication of how language is being used at a particular time," Baron said.
But the inclusion in Merriam-Webster of a singular "they" for non-binary individuals is a significant recognition that the new pronouns have reached the mainstream.
Baron, who is publishing a book next year called What's Your Pronoun?: Beyond He and She, said he knew the singular "they" was "here to stay" as soon as people started arguing about its correct grammatical usage. For example, should the reflexive form be "themself" or "themselves"? The question is still under debate.
"If people are already worried about how you express singular 'they' correctly, then it is already established," Baron said.
In recent months, a number of celebrities have come out as non-binary. Last week, the singer Sam Smith declared on Twitter that he has decided to go by the non-binary pronouns they/them, adding that "after a lifetime of being at war with my gender I've decided to embrace myself for who I am, inside and out".
"I understand there will be many mistakes and mis-gendering but all I ask is you please please try," Smith tweeted. "I hope you can see me like I see myself now."
The new definition of "they" was among 530 new words added to the dictionary, Merriam-Webster said on Wednesday. Among the other new words were "deep state", "dad joke" and "free solo".