Clade:
monophyletic group
Grade: non-monophyletic group, put together out of tradition or convenience, or to reflect morphologically distinct traits
Reptiles: grade (paraphyletic group)
Birds: clade
Mammals: clade
Clade vs. Grade
A + B
C + D
Sister Taxa
Sister Taxa:
two taxa (= named group of organisms) that are more closely related to each other than either is to a 3rd taxon, and derived from a common ancestral node.