What is the precise definition of clock skew in the design doc?

CockroachDB Design talks about clock skew. Does it mean the difference in time
shown by the clocks of the different nodes at the same absolute time? And the
design doc also talks about clock offset. Does clock offset mean the same thing
as clock skew?

Exactly, the design doc uses the terms “clock offset” and “clock skew” interchangeably to mean exactly what you said: “the difference in time shown by the clocks of the different nodes at the same absolute time.”

(We could perhaps be more precise about this, since “clock skew” means something different in electrical engineering. Feel free to open a PR!)

Not just electrical engineering: we try to use the definitions in RFC 1305

The uses of “clock skew” in the design doc should be updated to be “clock offset” instead.

Fixed in https://github.com/cockroachdb/cockroach/pull/18966.