What does "ad-hoc" mean?

Ad hoc means (as reported by the OED) "created or done for a particular purpose as necessary."

The discussions were on an ad hoc basis.
The group was constituted ad hoc.

The NOAD gives a similar definition: "formed, arranged, or done for a particular purpose only."
The Collins English Dictionary defines ad hoc as "for a particular purpose only; lacking generality or justification."

an ad hoc decision
an ad hoc committee

The phrase comes from Latin, and it literally means "to this."

made or happening only for a particular purpose or need, not planned before it happens

ad hoc committee/meeting

We deal with problems on an ad hoc basis (= as they happen).