Intergrown crystals: Mineral crystals that grow together like the pieces of a jigsaw puzzle as magma cools to form igneous rocks.

On regents exams the phrase 'intergrown crystals' almost always refers to igneous rocks although they may also be found in metamorphic rocks. The size of the crystals is an indication of how long it took for the rock to cool and may also indicate the location where the rock formed:

Large crystals (coarse texture) = slow cooling deep underground = intrusive
Small crystals (fine texture) = fast cooling on or near the surface = extrusive