A lens placed before the eyepiece to increase power of the eyepiece. 2x Barlow doubles, 3x triplets, etc.
A lens assembly with a negative focal length placed between an eyepiece and telescope that effectively increases the focal length (and magnification) of a telescope. Barlows commonly come in magnifications of 1.5x, 2X, and 3X.
Device which increases the focal length of an objective lens or mirror Effectively multiplies the magnification of any eyepiece. Common models include 2X, 3X, etc. Often used too aggressively by beginners. Also see focal reducer.