Learning about guitar and acoustic pickups and the physics and electronics behind them. I've heard that pyroelectric acoustic guitar pickups are nice? Looking and can't find anything.
You probably are hearing people say "piezoelectric pickup" because it is often erroneously pronounced "pie-zoh" instead of the correct "pee-ah-zoh".
Piezo (for short) pickups work by sensing vibrations. They are based around piezoelectric crystals which create electric potential (voltage) when they are flexed. They are used often for acoustic guitars because they pick up more the vibration of the top and body of the guitar versus a magnetic sound hole pickup, which works like an electric guitar magnetic pickup by inducing an electric current in the strings. Piezo pickups also work for nylon string guitars where magnetic pickups worn't work at all.
Piezo pickups are also popular for all kinds of other string instruments like violins and banjos, etc. And they are used for certain kinds of microphones. Some solid body electric guitars have piezo pickups build into their bridges to allow for a more acoustic sound from an electric guitar.
Piezo pickups have a crisp, often brittle sound, that can be musically appropriate but usually their sound quality is considered a downside of them. One very famous piezo pickup recording is the acoustic part of "Low" by Cracker from the 90s.
A pyroelectric pickup would convert fire into electric signals. It would likely only make sense for rather exotic playing styles.
You are probably thinking about a piezoelectric pickup. Those convert mechanical vibrations into electrical signals and are typically located in the bridge. They are more typical for acoustical guitars (and semiacoustics) than for electrical guitar since the usual magnetic pickups on an electric guitar fit the expected style of sound better.