A clade is a collection of taxa who share a collective taxonomic ancestor. These collections are also referred to as phyletic groups as they are used in phylogeny (hence their name, phyl-, rather than in reference to a phylum) to trace common ancestors, traits, and evolutionary history.
A clade may take one of three forms of phyletic groups. It is ideal and emphasized that a clade should be a monophyletic group. In this case, and in many cases across this site, the definition of a monophyletic group is synonymous with that of a clade.
However, a clade may also be paraphyletic or polyphyletic. These groups are not ideal and are often created by accident; they result from a lack of information and evidence.