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Posterior refers to something that is at the back or on the bottom, while anterior refers to something being in the front or on the top. Generally speaking, posterior and anterior refer to polar opposites. The terms are used most frequently in anatomy, but the terminology can also be used in zoology, anatomy and references to time, among other things.
Each of these two words can trace its roots to the Latin language. The first instance of the words being used is approximately 1535. The word posterior is a comparative of the word posterus, which means “coming after” and a derivative of the word post. The word anterior is a derivative of the word ante, which means “before.”
In zoology, these words typically refer to the tail and nose of certain organisms. In humans and some other animals, the tail is the buttocks and the nose is the head. To lay prone is to lay on one’s back, or posterior end. The anterior end of vertebrates that have very distinct heads might also be referred to as the rostral end — from the Latin for beak, usually used with birds — or the cranial end — from the Greek for brain — or cephalic end — from the Greek for head. In other organisms, posterior and anterior are usually replaced with the words ventral and dorsal, which would mean the belly and the spine.
In the medical world, the words posterior and anterior refer to the position of certain pieces of anatomy within the bodies of organisms. A gland that is anterior to another is in front of it. Anterior muscles would be muscles on the front or anterior side of the body.
Posterior and anterior can also refer to specific or non-specific points in time. An anterior event is one which comes before a certain date or time, just as a posterior event is one which comes after a certain date or time. So an anterior event precedes a posterior event. This application is typically used by referring to posterity, which means generations that follow. A common phrase is “for posterity’s sake,” which means for the sake of generations to come.
Although the words "posterior" and "anterior" may occur most frequently in medical terms, we can find them (or their derivatives) in every day speech.
The phrase "up the ante," which is used in context with gambling (most often poker), refers to increasing the original (or first) bet. This is used more commonly to mean increasing what is at stake in an uncertain situation.
"Posterior" itself is still used as a more polite and formal way of referring to a person's buttocks. And the original root, "post" is still a common addition to many words (such as postdate, postnatal) to mean after a fact or action.