Gravitational lensing is the astrophysical phenomenon whereby the propagation of light is affected by the distribution of mass in the universe. As photons travel across the universe, their trajectories are perturbed by the gravitational effects of mass concentrations with respect to those they would have followed in a perfectly homogeneous universe. The description of this phenomenon is similar in many ways to the description of the propagation of light through any other media, hence the name gravitational lensing or gravitational optics.
It is customary to distinguish three lensing regimes: strong, weak, and micro. Strong lensing is said to occur when multiple images of the source appear to the observer; this requires a perturber creating a strong gravitational field, and very close alignment of the lens and source. In general, the gravitational field of the deflector is not strong enough to create multiple images, and the observable effect is just a generic distortion of the images detectable only in a statistical sense; this is called weak lensing. For historical and practical reasons, strong lensing producing very small angular separations between the multiple images is called microlensing; the name comes from the typical angular separation created by the gravitational field of a star, which is typically of order microarcseconds.
Gravitational lensing is a relatively young field. Strong lensing, weak lensing, and microlensing observations are only a few decades old. Yet, in just a short time it has gone from a curiosity, mostly appreciated for its aesthetic and mathematical appeal, to a powerful tool used to study an impressive range of astrophysical phenomena, from planets to galaxies, clusters of galaxies, dark matter and dark energy.