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Webster 1913 Edition

Adj.

Of or pertaining to precaution, or precautions;

as,

precautionary

signals

Adj.

Containing previous caution; as precautionary advice or admonition.

Proceeding from previous caution; adapted to prevent mischief or secure good; as precautionary measures.

precautionary ‎(comparative more precautionary, superlative most precautionary)

Of, pertaining to, or serving as a precaution
The article 3.3 of the UNFCCC states the precautionary principle.

of, pertaining to, or serving as a precaution

precautionary ‎(plural precautionaries)

(obsolete) A precaution.
- 1748, Samuel Richardson, Clarissa
  Thou seest, Belford, by the above precautionaries, that I forget nothing.