key

An object designed to open and close a lock.

We tiptoed into the house, up the stairs and along the hall into the room where the Professor had been spending so much of his time. 'Twas locked, of course, but the Deacon man got a big bunch of keys out of his pocket and commenced to putter with the lock.

An object designed to fit between two other objects (such as a shaft and a wheel) in a mechanism and maintain their relative orientation.

A crucial step or requirement.

The key to solving this problem is persistence.

the key to winning a game

A small guide explaining symbols or terminology, especially the legend on a map or chart.

The key says that A stands for the accounting department.

A guide to the correct answers of a worksheet or test.

Some students cheated by using the answer key.

Indispensable, supremely important.

He is the key player on his soccer team.

Paradoxically enough, however, in general only the parties of the Left have done most to spread the belief that it was the numerical strength of the opposing material interests which decided political issues, whereas in practice these same parties have regularly and successfully acted as if they understood the key position of the intellectuals.

Important, salient.

She makes several key points.

Throughout the 1500s, the populace roiled over a constellation of grievances of which the forest emerged as a key focal point. The popular late Middle Ages fictional character Robin Hood, dressed in green to symbolize the forest, dodged fines for forest offenses and stole from the rich to give to the poor. But his appeal was painfully real and embodied the struggle over wood.

To fit (a lock) with a key.

To fit (pieces of a mechanical assembly) with a key to maintain the orientation between them.

To mark or indicate with a symbol indicating membership in a class.

So I worked on a tissue-paper copy of the perimeter plan, outlining groupings of plants of the same species and keying them with letters for the species.

The volume closes with thirty pages of "Notes, critical and explanatory," in which Thomson provides seventy-six longer or shorter notes keyed to specific sections of the synopsis.

To depress (a telegraph key).

To operate (the transmitter switch of a two-way radio).