sign - soil
- sign
- sign towing equipment
- signal
- signal area
- signal code
- signal determination
- signal intelligence
- signal pistol
- signal processing
- signal processing hardware
- signal processor
- signal reception
- signal reliability
- signal strength depression
- signal-to-interference ratio
- signal-to-noise ratio
- signalling
- signalling device
- signalling lamp
- signalling mirror
- signalman
- signature
- significant obstacle
- significant obstruction
- significant point
- significant weather
- significant weather chart
- significant weather forecast
- significant wreckage
- signing
- signing a maintenance release
- silent fan
- silent orbit
- silica-gel indicator
- silver
- simple approach
- simple approach lighting system
- simple-base propellant
- simplex
- simplex burner
