ordinal number

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

• noun A number indicating position in a series or order. The ordinal numbers are first (1st), second (2nd), third (3rd), and so on.

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

• noun grammar A word that expresses the relative position of an item in an ordered sequence.
• noun arithmetic A number used to denote position in a sequence.
• noun mathematics A generalized kind of number to denote the size of a well-ordered set.

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

• noun the number designating place in an ordered sequence