In the broadest sense, any system where the majority of capital (factories, firms, anything that is used to produce value) is controlled democratically or collectively.
I am personally a libertarian market socialist and believe most firms should be operated as worker cooperatives but that non-essential goods should still be distributed through a market system.
Socialism lost as a theory on how to run a successful nation. It was a lot of wishful thinking that lured many a person into its siren call of utopia, kind of like libertarians.
We are not transforming to that any time soon, unless the world collapses and resets, and then it's a roll of the dice on who gains power back and what they do with it.
We might nationalize an industry or two more, but that's about it I would think, we're in pretty good shape except for a few caveats.
It's funny, you look at China, the largest "socialist" country in human history, and even they don't agree on what is socialism, or whether they really are socialist, or really care about socialism. They have steadily added capitalism, citing "we use what works" instead of classifying it as socialist or capitalist, and they see the impact the United States, using capitalism mixed with social programs, has had, and they envy it. They recognized the weaknesses early on. They still embrace the concept, but not necessarily practice it. Having a one party system that also controls the military, is just scary AF.
The U.S. can benefit from separate corporate/wealth power from government even more, if we can just get Republicans to wake up from their right wing propaganda stupor.
And we can use public health insurance (at the least), and perhaps some increase in public funding of post high school education (I think trades and college both), and some worker benefits that are sorely needed. But aside from healthcare, this is mostly fine-tuning.
We don't know what the future holds...no one really does, this is kind of new territory for the human race.
It will be interesting if we develop AI to help solve national problems, it might be a new style of governing, that includes AI solution sets, which we then analyze and prune, and then vote on.