Definition of acidic

1. Said of an igneous rock with a silica content above 63 weight-%
(IUGS recommendation for the classification of igneous rocks)

2. A descriptive term applied to those igneous rocks that contain more than 60% silica. Acidic is one of four subdivisions of a widely used system for classifying igneous rocks based on their silica content: acidic, intermediate, basic, and ultrabasic.

Ref: AGI

3. Applied loosely to any igneous rock composed predominantly of light-colored minerals having a relatively low specific gravity.

Compare with: felsic