a spherical red or purple gall which forms on the leaves or male catkins of oak trees in response to the developing larva of a gall wasp. It results from eggs laid in the spring and alternates within the annual cycle with the spangle gall.
The wasp is Neuroterus quercusbaccarum, family Cynipidae
currant gall in other Oxford dictionaries
Definition of currant gall in the US English dictionary