ion channel

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

• noun A pathway through a protein molecule or complex in a cell membrane that modulates the electrical potential across the membrane by controlling the passage of small inorganic ions into and out of the cell.

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

• noun biochemistry a protein complex or single protein which penetrates a cell membrane and catalyzes the passage of specific ions through that membrane.