In astrology, an orb is a level of tolerance in the measured angle between two astral items.
For example, a trine is formed when there is an angle of 120° between two items. Yet, the measured angle will rarely be precisely 120°. Therefore, astrologers allow for a tolerance. For trines, it can be set at 4°. Hence, if the measured angle between two items is between 116° and 124°, it will be considered as a trine aspect.
Sometimes, astrologers define orbs per astral item, rather than per aspect. For example, if one of the astral item is the Sun, they may allow for an other orb, say 10°. Then, instead of using an 4° orb for trines where one of the item is the Sun, an orb of 10° will be used instead. The tolerance becomes 110° to 130°.
An orb can also be defined per combination of an astral item and an aspect. The rule becomes "if we are dealing with a square and if one of the items is Saturn, then the a 7° orb applies". It would not be applied for other aspects (opposition, sextile...) or if none of the astral item is Saturn.