hardening off
n.— «It is best not to put seedling so young directly out in the garden. “Hardening off” is the term for gradually acclimating them to outdoor conditions. I use an half-way house, also known in garden circles as a cold frame, i.e., an enclosed, bottomless box with a clear glass or plastic lid that can be propped open for ventilation or closed against a chilly night.» —“Gardener to tomatoes: Step outside!” by Valerie Sudol Star-Ledger (New Jersey) May 6, 2008. (source: Double-Tongued Dictionary)
Further reading
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This is not a new term. I first heard and read it in the early 1980’s and it is probably older than that.
This meaning is found in the Collins online dictionary.
http://dictionary.reverso.net/english-definitions/harden%20off