down payment

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

• noun A partial payment made at the time of purchase, with the balance to be paid later.

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

• noun law A payment representing a fraction of the price of something being purchased, made to secure the right to continue making payments towards that purchase.
• noun By extension, any initial commitment signifying an intention to carry out a larger future commitment, even though no legal rights or obligations are secured.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a partial payment made at the time of purchase; the balance to be paid later