impleader

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

• noun The procedure by which a third party is impleaded.

from The Century Dictionary.

• noun One who impleads or prosecutes another; an accuser; an impeacher.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Law) One who prosecutes or sues another.

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

• noun law A procedural device before trial in which a party joins a third-party into a lawsuit because that third-party is liable to an original defendant.