One of the three Modalities. Cardinal signs are: Aries, Cancer, Libra, Capricorn. Cardinal traits are: goal-oriented, purposeful, ambitious, leadership, self-starting, opportunistic, restless when not focused on ambitions.
Outward movement - Aries, Cancer, Libra and Capricorn - enterprising or goal-oriented.
The four basic Elements (Fire, Earth, Air, and Water) can each act in one of three basic ways, or Modes (Cardinal, Fixed, or Mutable). In addition to the above links, the Study Guide Signs of the Zodiac, Houses, and Planetary Rulers shows the Mode and Element of each sign. This seems very complicated at first, but once you have grasped the concept the symbolic language of Astrology can be experienced as an elegant language! Each single symbol or glyph carries so much information.