Probably the best answer is whatever Bernie Sanders says it is as he is by far the most famous person ever to adopt this term as a label for his beliefs. There is a group in the US bearing that name, the Democratic Socialists of America (DSA), which has been in existence since 1983. But while its membership has since then generally fluctuated between 4,000 and a bit over 6,000 through 2016, its membership had surged to over 45,000 by 2019, clearly responding to Bernie’s identification with the term, even though as near as I can tell, he has not been a member of the DSA.
If one goes to the Wikipedia entry on “democratic socialism,” one finds claims that it originated with the utopian socialists and chartists in the early and mid-1800s. Certainly many of these groups supported democracy and also some sort of socialism. For that matter, Karl Marx also in many writings supported democracy and socialism, although in other places Marx sneered at what he called “bourgeois democracy,” and we know many regimes claiming to be inspired by Marx have not been democratic, unless one wants to call Leninist “democratic centralism” to be democratic, something most of us would not go along with, and current DSA types would not go along with.