S
- S0 Galaxy
- Scale Height
- Scale Length
- Scaling Relations
- Scattered Disk Objects
- Schwarzschild Lens
- Schwarzschild Radius
- Seasons
- Second
- Secular Evolution
- Self-gravitating Bodies
- Self-gravitation
- Semi-major Axis
- Semi-minor Axis
- SEST
- SETI
- Shapiro Delay
- Shells
- Shooting Star
- Short-period Comets
- SI Units
- Sidereal Day
- Sidereal Period
- Single Stellar Populations
- Singularity
- Six-Degree Field Galaxy Survey
- Small Magellanic Cloud
- Soft Gamma Repeater
- Soft X-ray Transient
- Solar Cosmic Rays
- Solar Day
- Solar Flare
- Solar Mass
- Solar Wind
- Solid Angle
- Solstice
- South Galactic Pole
- Spectral Classification
- Spectral Continuum
- Spectral Line
- Spectral Line Broadening
- Spectral Line Profile
- Spectral Type
- Spectroscopy
- Speed