Gravitational microlensing (or just microlensing) is gravitational lensing by smaller objects, e.g., by stars within a galaxy rather than by an entire galaxy. (The term also applies to lensing by objects smaller than stars, e.g., planets, but the term nanolensing has been used specifically for this regime.) From it, some information can be gleaned about the lensing and lensed objects. Most observable instances show nothing but a magnification of the light of the lensed object, the angle of the light's redirection being an immeasurable tiny fraction of an arcsecond. They are detected by the light curve which occurs as one object passes in front of the other (a microlensing event). Among their uses have been surveys aimed at finding MACHOs lensing stars, and the study of distant active galactic nuclei lensed by stars.
The observed phenomena can be described in relation to the event's Einstein radius (aka Einstein angle), which is the angular radius of an Einstein ring, the angular radius of a ring of light that would be formed if the two were exactly aligned, and can be estimated by the length of the event. It is also related to the mass of the lensing object and the distances to both objects and if two of these can be determined by other means, the other can be calculated based upon the event. For example, also having "normal" parallax measurements of both stars (if both are close) allows the mass of the lens to be determined.
Several factors affect the pattern observed: the position of the observer (termed an Einstein parallax: either the movement of Earth, or comparison with an observation from a distant space observatory potentially yields this), the position of the lensed object, e.g., if it is one of a binary star pair with significant orbital motion during the event (producing xallarap, a parallax-like measurement due to the displacement of the lensed object rather than the observer; the name coming from parallax spelled backward), or an extra-solar planet orbiting the lens, i.e., an additional small lens, or significant size of the objects causing a divergence from the typical analyses that model them as point sources. All these effects can be used to glean more information, yielding additional equations that can be used to break the degeneracy and yield stellar parameters.