Step-down converters is really a subclass of DC-DC converters, while a buck converter is one specific topology ("brand") of step-down converter. In essence, a buck converter is a step-down converter, but not every step-down converter is a buck converter. In theory, anyway.
Let's look at an overview of the various non-isolated DC-DC converter topologies:
As you can see, the subclasses are step-up, step-down, and step-up/down, and there are some topologies in each subclass. You can also see that there's only one topology in the step-down subclass: the buck converter. So:
But, why is a step-down converter called a buck converter?
Because buck converters are step-down converters, and in practice all step-down converters are buck converters. The names refer to different things, but in practice it doesn't matter, so they're used interchangeably.
The Texas Instruments book Power Topologies Handbook (by Markus Zehendner and Matthias Ulmann) also has a good overview of the various topologies (also including isolating converters).
If you want to know where the name "buck converter" comes from; I don't know, but the other answers try to address that.