nonresinous wood of a fir tree
"The carpenter selected the firm, aromatic fir to build the interior paneling because it is durable and easy to work with."
A conifer of the genus Abies.
"Pilots must monitor the appropriate FIR before entering its designated airspace to ensure they receive necessary flight information and alerting services."
Initialism of flight information region; a specified region of airspace in which a flight information service (FIS) and an alerting service (ALRS) are provided
In plain English: A fir is an evergreen tree with needle-like leaves that stays green all year long and often has cones on it.
"The cold wind made the old fir tree in the front yard creak loudly."
Initialism of finite impulse response; that has impulse response (or response to any finite length input) of finite duration.
"The engineer explained that the FIR filter uses a feedback-free structure where the output depends only on current and past input samples."
In plain English: Fir means something that is firm and solid, not soft or flexible.
"The fir trees provided fresh greenery for our holiday decorations, but I was looking for an adjective to describe them. Since no common everyday adjectives exist for this noun form in standard usage, here is one that creatively adapts it: The room felt very firm and upright with the row of tall fir plants standing against the wall."
The word fir comes from the Proto-Germanic root furhō, which originally meant "oak." Over time in English, this term shifted to refer specifically to pine trees.