The zodiacal sign in which the moon is located, determined by the moment and location for which a horoscope is drawn (such as the time and place of birth). Many astrologers believe the moon rules emotions (and rules the intuition along with Neptune); therefore, the Moon sign in a birth chart will reveal a great deal about a person's personality. 

Many astrologers believe the three most significant parts of a birth horoscope are the Sun sign, Moon sign, and ascendant.