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The way people talk about "capitalism" is, perhaps unsurprisingly, not very clarifying. At times it seems to mean simply "having an economy at all," then it skips over to being something really specific (though unspecified) that we need to defend.
In reality, capitalism isn't about trade, or the use of money, or allocating scarce resources -- it's a particular economic arrangement oriented toward maximizing the accumulation of capital. That's why it's *capital*-ism. Ever-greater capital is the axiomatic goal.
You could imagine forms of trade that are about getting resources where they need to go and compensating the traders for their time and effort, *without* an open-ended accumulation of ever-greater piles of money. That would be most of human history.
Similarly, money has been used as an economic lubricant, unit of account, and store of value from time immemorial *without* necessarily entailing an open-ended accumulation of ever-greater piles of money. That's what's unique about capitalism.
We could also imagine an open-ended accumulation of ever-greater piles of money that was just a game, not tied to the immiseration of human beings and the squandering of finite natural resources. But that game would have to be contained in a system with non-accumulative goals.
So to review, capitalism is *capital*-ism -- the open-ended growth of capital, ultimately by any means necessary, is the goal of the system, the axiomatic value that orders all other values. It arose in history and will end in history (if only by bringing about our extinction).
(It might also be worth noting that the term "capitalism" was coined by leftist critics of the modern economic system and popularized by Karl Marx, as a way of pushing back against political economists who put forth the modern system as natural or inevitable.)
