The best replacement for key is legend.

The best alternatives to key are rated as following:
★★★★★  legend
★★★★★  island
★★★  foreign key
★★  cay
★★  private key
★  primary key
★  index
★  pitch
★  unique key
★  microsoft natural ps 2 keyboard
★  password
★  passphrase
★  unique index
★  induced fit
★  legend of a map
★  wpa key
★  shared key
★  open system
★  certificate
★  siesta key
  pbx
  surrogate key
  lapel pin
  scale
  sanibel island
  product id
  combination bike lock
  low key
  88 key
  clustered index
  minor key
  index mysql
  candidate key
  allen wrench
  induced fit hypothesis
  legend on a map
  master code
  keyed alike
  master key
  public key
  natural key
  unique
  mode
  combination lock
  octave
  chroma key
  clearwater

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Speaking about key, one probably shouldn't forget about key.

Other search terms related to key and their ratings are:
★★★★★ key
★★★★ reinstalling
★★★ reinstall
★★★ passphrase
★★ spare
★★ door
★★ fn
★ windows keyboard
★ push button ignition
★ primary
★ quotes
index
b and e flat
primary key
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Google Trends

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key
legend
island