In astrology the term orb is used to refer to a range of influence in which the aspect or configuration between two planets is thought to come into effect.
Aspects are usually defined in terms of a geometrical relationship between two planets, and an exact aspect is when the two planets form a geometrical relationship that involves the precise number of degrees that are associated with that aspect. For example, a an exact trine is when two planets are exactly 120 degrees away from each other in the zodiac. An exact square is when two planets are exactly 90 degrees away.
Although the specific meaning associated with an aspect is thought to be the most acute when the two planets are configured by the exact number of degrees associated with the aspect, there is a range of degrees both before and after the exact aspect in which the effect of the aspect is still thought to be felt. For example, although an exact trine is 120 degrees, the trine is also thought to be in effect already when the two planets are only 119 degrees apart, as well as when they are 121 degrees apart. Or in other words, both before and after the exact configuration.
Definition of Orb
This is where the concept of an “orb of influence” comes in, which is usually just shortened and referred to as an “orb”. The orb of influence defines the range of degrees before and after the exact aspect in which the effect of the aspect is thought to extend to.
The orb defines how close to the exact aspect a planet must get in order for the effect of the aspect to begin, as well as how far away from the exact aspect a planet must get in order for the effect of the aspect to end.
Orbs are usually given in a certain number of degrees for each planet. Sometimes it varies from planet to planet. For example, Sakoian and Acker recommend an orb of 6 degrees for major aspects between planets, but 10 degrees for aspects involving the Sun and Moon (Sakoian and Acker, pg. 81).
Astrodienst’s default table of orbs, which they say are used by astrologer Liz Greene, give a 10 degree orb for any configurations by conjunction, square, trine and opposition, but only a 6 degree orb when the planets involved are configured by sextile.
Minor aspects are generally given very tight orbs of 2 or 3 degrees or less.
Although the concept of a planetary orb is widely used, there is no generally agreed upon standard for orbs, and thus a lot of variation exists in the tradition when it comes to specifying the exact number of degrees that should be involved.
Sakoian, Frances and Louis S. Acker. Predictive Astrology. Harper & Row. New York, NY. 1977.