In telecommunication, an index-matching material is a substance, usually a liquid, cement (adhesive), or gel, which has an index of refraction that closely approximates that of an optical fiber, and is used to reduce Fresnel reflection at the fiber endface. (FAA)

Note 1: An index-matching material may be used in conjunction with pairs of mated connectors, with mechanical splices, or at the ends of fibers.

Note 2: Without the use of an index-matching material, Fresnel reflections will occur at the smooth endfaces of a fiber. These reflections may be as high as -14 dB (i.e., 14 dB below the level of the incident signal). When the reflected signal returns to the transmitting end, it is reflected again and returns to the receiving end at a level that is (28 plus twice the fiber loss) dB below the direct signal. The reflected signal will also be delayed by twice the delay time introduced by the fiber. The reflected signal will have no practical effect on digital systems because of its low level relative to the direct signal; i.e., it will have no practical effect on the detected signal seen at the decision point of the digital optical receiver . It may be noticeable in an analog baseband intensity-modulated video signal.

Source: from Federal Standard 1037C