noun

• (chiefly countable) A conifer of the genus Abies.
  "A beech wood with silver firs in it rolled down the face of the hill, and the maze of leafless twigs and dusky spires cut sharp against the soft blueness of the evening sky."
• (chiefly countable) Any pinaceous conifer of related genera, especially a Douglas fir (Pseudotsuga) or a Scots pine (Pinus sylvestris).
  "we shall find a spot that is sheltered and snug enough, sir. There is a dry fir-wood just ahead, if I remember rightly."
• (uncountable) Wood of such trees.

adj

• (signal processing, of a filter) Initialism of finite impulse response; that has impulse response (or response to any finite length input) of finite duration.

noun

• (aviation) Initialism of flight information region; a specified region of airspace in which a flight information service (FIS) and an alerting service (ALRS) are provided.
• (South Asia, Singapore, law enforcement) Initialism of first information report.
• (uncountable) Abbreviation of far infrared or far infrared radiation.