The prefix peri- means “around” or “near.” It comes from ancient Greek and helps build words that describe things surrounding, encircling, or being close to something else.

In perimeter, you can see peri- combined with meter (measure): the perimeter of a playground is the boundary that goes around it, the total distance if you walked along all its edges. In periscope, the tool submarines use to see above water, peri- suggests looking around from a hidden position.

The prefix shows up in many scientific and medical terms. The pericardium is the protective sac around your heart. When doctors talk about the perinatal period, they mean the time around a baby's birth, including the weeks just before and after.

You might also encounter peri- in astronomy: perihelion describes when a planet or comet comes nearest to the sun in its orbit. Even the word period itself contains this prefix, which makes sense whether you're talking about a period of time or the period at the end of a sentence that signals you've come full circle to complete a thought.