There are so many dozens of interpretations of class now - let alone historically - that it's hard to define. In some Marxist writing, "working class" refers to anyone who is basically salaried, doing waged work. Versus the "bourgeoisie", who own the means of production (capital and land) and could coerce others into working.
Of course, this is complicated now by the existence of things like shareholders, who can be both, or buy-to-let landlords who also have a waged job, etc. etc. etc. Also, by the rise of the service industry, which means "means of production" would have to be extended to things like warehouses with telephones in them, I guess.
Sociologically, of course, there are many other definitions of class- based on a whole range of indicators including consumption, mores, education, even social media usage.
And then there are those like me who'd make a case that class is really about how we talk about class, discursively, at any given moment in time, and that this is about far wider debates about how the reality of economics is interpreted at a given moment, given a given configuration of power relations. 