There are as many definitions of astrology as there are astrologers out there. Here's an evolving collection from those in the field.
Nick Campion, from The Ultimate Astrologer: Astrology is the word of the stars - from the Greek astra (star) and logos (word). It is the study by which we read significance for our lives into the movements of the stars and planets.
Jessica Murray, from Soul-Sick Nation: The interplay of various planetary cycles has been found to have the qualities of a clock: an astoundingly intricate, mysteriously precise clock. Astrology has organized the workings of this clock into symbolic patterns, from which we can infer meanings about "what time it is."
Kevin Burk, from Understanding the Birth Chart: Astrology is the study of cycles. By observing the cyclical movements of the planets, we are able to gain a greater understanding of the cycles and patterns in our own lives.
Cafe Astrology: Astrology is, put simply, the study of the correlation between the astronomical positions of the planets and events on earth.
Musawa of Mother Tongue Ink: Astrology measures the cycles by relating the Sun, Moon and all other planets in our universe through the star signs (the zodiac), helping us to tell time in the larger cycles of the universe.
Fred Gettings, from The Arkana Dictionary of Astrology: The study of the relationship between the macrocosm and the microcosm, which (in materialistic terms) is often defined as the study of the influence of the celestial bodies on the Earth and its inhabitants.
Astrodienst: Astrology sees mankind as being not only influenced by hereditary factors and the environment, but also by the state of our solar system at the moment of birth
Adrian Ross Duncan, from Astrology; Transformation and Empowerment: The basic principle of astrology is that the smallest thing in the universe is subject to the same process as the largest. The same rules apply for both, and indeed an action in one sphere will reflect an action in the other. Astrology is unique in that it applies rules of correspondence between time and space, linking them to human character and history.
Dane Rudhyar, on astrology: ...The first known attempt to discover a consistent and dependable order in existence produced the earliest and most basic forms of what is today called astrology. As such, astrology can truly be said to be the "mother of all sciences" and the original core of all culture and religions.
Vicki Noble, from Shakti Woman: Ancient astrology was body based and biological. In the beginning it was a felt experience, but after the beginnings of patriarchal civilization, it became a "science" in the more modern sense of the word. Shamanism and astrology were not separated until very late in our evolution, when the separation from Nature and our bodies was made.
Stephen Arroyo, from the Chart Interpretation Handbook: These five factors --Elements, Signs, Planets, Houses and Aspects -- comprise a comprehensive cosmic psychology, and it is the art of combining them that results in the language of energy called astrology.
Eileen Grimes, from Titanic Astrology: Each of the planetary energies (and we include the Moon's energy in this group) plays a part in forming our entire, integrated persona. Taking the sum of the parts can quite accurately show us who we are, as well as how others see us. Astrology helps us understand how all the parts of ourselves work together within our life and psyche.