macronucleus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The larger of two nuclei present in ciliate protozoans, which controls nonreproductive functions of the cell, such as metabolism.

from The Century Dictionary.

• noun A large nucleus which may subdivide into or be replaced by smaller nuclei.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology The larger of the two nuclei present in ciliate protozoans; it controls the nonreproductive functions of the cell.