In astrology, a cardinal sign (also called by older astrologers a moveable sign) is a sign of the zodiac that initiates a change of temperate zone season when the Sun makes its annual passage into them.
The word “cardinal” originates from the Latin word for “hinge,” since they each mark the turning point of a temperate season. They were called moveable by traditional astrologers because, as Bonatti says, the “air” changes when the Sun enters each of these signs, bringing a change of season. For this reason, these signs are considered forceful and dynamic, initiating action and providing leadership. These signs tend to direct their energy outwards.
On the other hand, cardinal signs can also be inconsiderate, controlling and demanding, as they feel their way is what’s best for others to follow.
Sometimes the word cardinal is confused with the word angular. Angular signs are those signs which are located on the astrological angles of any given natal chart. Angular houses may be cardinal, fixed or mutable, depending on the birth time of the chart, but only Aries, Cancer, Libra and Capricorn are cardinal signs.
Definition courtesy Wikipedia.