Here's the definition of the word taken from macOS's built-in dictionary (New Oxford American Dictionary):
> leftist, noun: a person who has left-wing political views or supports left-wing policies.
> left-wing, noun: the section of a political party or system that advocates for greater social and economic equality, and typically favors socially liberal ideas; the liberal or progressive group or section
Anti-capitalist thought represents a subset, but is certainly not the sole marker of left-wing political thought.
Now, my _personal_ take on this is that western left-wing thinking and liberalism (in the moral philosophical sense) are deeply intertwined (given the Enlightenment and its values), but anti-capitalist thought is deeply illiberal in nature. It is this fundamental contradiction that leads to permanent infighting within the left-wing spectrum.