But what's at one end of the spectrum here?

Light is a spectrum, and the ends of that spectrum are defined: 0 frequency at one end and infinite frequency at the other. A spectrum covering an infinite number of points ranging between two precise, simple definitions.

I define socialism as: joint ownership of the means of production by the people

I like the definition of capitalism as this, from wikipedia:

Capitalism is an economic system based on the private ownership of the means of production and their operation for profit. Central characteristics of capitalism include capital accumulation, competitive markets, price systems, private property, property rights recognition, voluntary exchange, and wage labor.

My short snarky definition is: Capitalism is the economic system where cooperation requires consent.