step stool

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

• noun A stool, often with folding steps attached, on which one stands to reach high objects.

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

• noun A stool of various designs that may have one or more steps that fold away underneath the seat when not in use.

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

• noun a stool that has one or two steps that fold under the seat