The word key has several meanings:

1. A small metal tool that opens a lock. When you turn a key in a lock, tiny metal pins inside align perfectly to let the bolt slide open. Before keys were invented, people used heavy wooden bars or complicated mechanisms to secure their doors. Today, keys protect everything from houses to cars to safes. Many people carry a key ring with multiple keys for different locks.
2. Something crucial or essential to success. When a coach says teamwork is key to winning, she means the team can't succeed without it. A student might identify the key concept in a chapter, meaning the main idea that helps everything else make sense. Scientists search for key evidence that supports their theories.
3. The buttons you press on a keyboard or piano. When you type, you press the letter keys. When you play piano, each key produces a different note. A keyboard gets its name from these individual keys.
4. In music, a system of notes that work together. A song written in the key of C uses certain notes that sound harmonious together. Singers talk about songs being in a high key or low key, meaning whether the notes are higher or lower overall.