The oscillatory motion of two bonded atoms relative to each other. This motion consist of the two bonded atoms stretching passed their equilibrium position, then returning to their equilibrium position, and finally contracting passed their equilibrium position. The result is an increase in frequency of the bond which allows it to absorb light at that same frequency.
[Baird, Colin. Environmental Chemistry. New York: W.H. Freeman, 1999.]
Source: Atmospheric Chemistry Glossary