Capitalism has historically been defined by its opponents rather than its proponents, and in terms of its consequences rather than its mechanisms. Specifically, it tends to get defined primarily in terms of its intended malicious effects and unintended/unaccounted damaging effects: oppression and social costs.
To navigate by such definitions is to deeply misunderstand the nature of the beast, so I want to propose a definition of capitalism (or if you prefer a less loaded term, commerce) in terms what I will argue is its essential mechanism:
Capitalism is the indirect manipulation of illegible human relationships, through the peaceful manipulation of decision contexts.
In practice, capitalism often operates with directness and violence, but the point of this definition is to get at the sine qua non that distinguishes it from other societal mechanisms. The role of “capital” is actually peripheral, since capital plays a role in defining the mechanisms of politics, kinship and war too.
The central fact about the mechanism of capitalism is that it needs neither capacity for direct action on a system, nor violent means, to influence it. This is not just a claim about an idealized model of capitalism: it is a claim about the real thing. Capitalism in our world has arguably gotten less direct and less violent over time. Today the visible motif that best captures the spirit of capitalism is not the East India Company warship, but advertising: messaging mechanisms that work to shape the background context of decisions.
Let me attempt an exegesis of my definition with a parable about the It Couple of geekdom, Alice and Bob.