This refers to the base pitch of the instrument. refers to the tuning of an instrument, and the number is the type of tuning. Each number represents a semitone increase above the base key of low F, with F being 1. So if you have a , you have a Bb flute (F F# G G# A Bb (A#)). The others change accordingly.
On a side note, there is a word that carries the meaning "monotonous" in the sense that we use it in English, as in a monotonous (or boring) speech.
This site has some really good explanations if you want more information.