A seesaw (also known as a teeter-totter or teeter board) is a long, narrow board pivoted in the middle so that, as one end goes up, the other goes down.

In a playground setting , the board is balanced in the exact center. A person sits on each end and they take turns pushing their feet against the ground to lift their end into the air. Playground seesaws usually have handles for the riders to grip as they sit facing each other. One problem with the seesaw's design is that if a child allows himself/herself to hit the ground suddenly after jumping, or exits the seesaw at the bottom, the other child may fall and be injured. For this reason, seesaws are often mounted above a soft surface such as foam or wood chips.

Seesaws, and the eagerness of children to play with them, are sometimes used to aid in mechanical processes. For example, at the Gaviotas community in Colombia, a children's seesaw is connected to a water pump.

In the United States a seesaw is also called a "teeter-totter". However, most commonly a "teeter-totter" is a two-person swing on a swing set, on which two children sit facing each other and the teeter-totter swings back and forth in a pendulum motion. According to linguist Peter Trudgill, this term originates from the Norfolk language word tittermatorter. Both teeter-totter (from teeter, as in to teeter on the edge) and seesaw (from the verb saw) demonstrate the linguistic process called reduplication, where a word or syllable is doubled, often with a different vowel. Reduplication is typical of words that indicate repeated activity, such as riding up and down on a seesaw.

The outdoor toy usually called a seesaw has a number of regional names, New England having the greatest variety in the smallest area. In southeast New England it can be referred to as a tilt or a tilting board. Speakers in northeast Massachusetts have been known to call it a teedle board; in the Narragansett Bay area the term changes to dandle or dandle board. These regional names are not very common, and have become antiquated. Children call it a seesaw more likely than not in Massachusetts. Teeter or teeterboard is used more generally in the northeast United States, while teeter-totter, probably the most common term after seesaw, is used across the inland northern states and westward to the West Coast.

According to Michael Drout
There are almost no "Teeter-" forms in Pennsylvania, and if you go to western West Virginia and down into western North Caroline there is a band of "Ridey-Horse" that heads almost straight south. This pattern suggests a New England origin or importation of the term that spread down the coast and a separate development in Appalachia, where Scotts-Irish settlers did not come from New England. "Hickey-horse" in the coastal regions of North Carolina is consistent with other linguistic and ethnic variations.

Mechanically, a seesaw is a first class lever. The simple mechanics of a seesaw make them appear frequently in school exam paper questions on mechanical problems.