magnifying glass

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

• noun A lens or combination of lenses that enlarges the image of an object.

from The Century Dictionary.

• noun In optics, a convex lens: so called because objects seen through it have their apparent dimensions increased.

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

• noun A single convex lens which magnifies the apparent dimensions of objects seen through it, and is used to produce an enlarged image.

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

• noun An instrument made of convex glass, used to magnify.

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

• noun light microscope consisting of a single convex lens that is used to produce an enlarged image