For the record, we are all incredibly unique and complex souls. That said, some of us are also extremely one thing, whether that one thing is competitive, creative, or shy. If you've ever wondered why one of your personality traits seems to outshine the rest, it's high time you checked your birth chart — you just might have a stellium.
A stellium, known in older forms of astrology as a satellitium, is simply a cluster of three (or, if the moon or sun are involved, four) or more planets in a single sign or house. Having a stellium in your natal chart means your life will be heavily influenced by the traits commonly associated with that sign or house, particularly in the areas your stellium planets rule.
Maybe you were born at a moment when Mercury, Mars, and Neptune all happened to be in Capricorn. That means you approach matters of communication, action, and spirituality very much like a Capricorn (read: you probably choose your words carefully, you're very impatient, and you almost definitely worship logic over any other deity). In other words, with so many planets hanging out in Capricorn, you're bound to exhibit quite a few Goat-like traits.
Of course, having a stellium doesn't automatically reduce your personality to a single note. (Remember, we are all beautiful, complex beings.) And, as astrologer Donna Cunningham writes, a stellium is simply a major area of focus in your life, which may manifest as a challenge to overcome or a guiding force, rather than a behavioral death sentence. Observing how your stellium influences your decisions can indicate what you're really passionate about. For instance, it'd make perfect sense if you had the moon, Venus, and Mars all in Pisces and you sought out emotional honesty and creativity in your partners.
Discovering that you have a stellium can also help you understand your personality on a deeper level, especially if you've never really identified with your sun sign (a.k.a. the sign that the sun was in at the moment of your birth and the sign that most horoscopes are associated with). There's a good chance that your behavior more closely resembles the sign that has the stellium.
Learning about any potential stellia you may have in your birth chart might not change how you read your regular horoscope completely, but it will give you a better idea of your complete astrological makeup. And, the next time someone asks you why you're just so much like an Aries, you'll have an answer.