In astrology, the zodiacal sign in which the Sun could be seen at the time of birth. When someone asks, "What's your sign?" they actually are asking for your Sun sign.
While the Sun is certainly an important planet in a horoscope, it has been stressed in simplistic books and in newspaper astrology columns, giving many people the idea that Sun Sign astrology is the entirety of astrology, and a straw man often used by debunkers and pseudo-skeptics against astrology.
To astrologers, the Sun is considered to indicate the Outer Self that a person reveals to the world; their apparent personality. In contrast, the Ascendant in a horoscope is believed to indicate the "True Self" or inner self.
While much can be revealed by knowing the Sun sign, the Sun is merely one of the planets, asteroids, and other points used in a horoscope. Along with the Sun sign, the Moon sign, and ascendant are considered to be the three most important elements in a birth chart.