The zodiac is a ring of twelve signs, and this is number eight in the ring. Its picture is a scorpion. The sign is really a stretch of dates: if you were born in that stretch, this is your sign. Some people believe the sign affects your character and your life; other people read it for fun and believe none of it. The word says nothing about whether it is true. When you talk about the sign itself the word is uncountable, so you say my sign is Scorpio, not my sign is a Scorpio. The exact dates move by a day or two from one source to another.

Here the sign's name is used for the person. If your birthday sits in that stretch of dates, you are a Scorpio. This one is countable: a Scorpio, two Scorpios, all the Scorpios in the room. That is the quickest way to tell the two uses apart — an a or a plural -s in front of your eyes means a person is meant. People use it to introduce themselves, to sort friends into groups, and to joke about what someone is like. Saying it does not commit you to believing any of it.