A 45 rpm adapter is a small plastic or metal insert that goes in the middle of a 45-rpm record

Gramophone record

A gramophone record, commonly known as a phonograph record , vinyl record , or colloquially, a record, is an analog sound storage medium consisting of a flat disc with an inscribed, modulated spiral groove...

 so it will play on a turntable

Turntable

A turntable is generally a rotating platform, and may refer to:-Music:* Turntable, a motor-driven platform that normally rotates a gramophone record at a constant rotational velocity as part of a phonograph....

. The adapter could be a small solid circle that fits onto the bottom of the spindle (meaning only one 45 could be played at a time) or a larger adapter that fits over the entire spindle, permitting a stack of 45s to be played.

The "Spider"

The former RCA Corporation

RCA

RCA Corporation, founded as the Radio Corporation of America, was an American electronics company in existence from 1919 to 1986. The RCA trademark is currently owned by the French conglomerate Technicolor SA through RCA Trademark Management S.A., a company owned by Technicolor...

 introduced a snap-in plastic insert known as a spider to make 45 rpm records compatible with the smaller spindle size of a 33⅓ rpm LP record player. Commissioned by RCA president David Sarnoff

David Sarnoff

David Sarnoff was an American businessman and pioneer of American commercial radio and television. He founded the National Broadcasting Company and throughout most of his career he led the Radio Corporation of America in various capacities from shortly after its founding in 1919 until his...

and invented by Thomas Hutchison, spiders were prevalent in the 1960s and sold tens of millions per year.