Definition of "hotbed"

hotbed

noun
plural hotbeds
1. A low bed of earth covered with glass, and heated with rotting manure, used for the germination of seeds and the growth of tender plants, like a miniature hothouse.
  Quotations
  Tomato seedlings will be ready to transplant from the hotbed to the coldframe in 12 to 18 days after sowing the seed, depending upon the amount of sunshine and the care given the bed.
  1924, Erval Jackson Newcomer, M. A. Yothers, Warren Draper Whitcomb, Control of the Codling Moth in the Pacific Northwest, page 11
2. (by extension) An environment that is ideal for the growth or development of something, especially of something undesirable.
  Quotations
  The problem faced by Government and railway alike is the dreadful legacy of the 'stay home' policy which has left millions of people fearing that trains are a hotbed of infection to be avoided like the... well, do I have to say it?!
  2020 August 12, Nigel Harris, “Comment: Rail at the brink? Not yet...”, in Rail, page 3
3. An iron platform in a rolling mill, on which hot bars, rails, etc., are laid to cool.