L
- Large-scale Structure
- Last Quarter Moon
- Late-type Galaxies
- Leap Year
- Length
- Lenticular Galaxy
- Lepton
- Light
- Light Year
- LIGO
- Limiting Magnitude
- Linearly Proportional
- Local Flow
- Local Group
- Local Noon
- Long Baseline Array
- Long-period Comets
- Lookback Time
- Loose Groups
- Lovell 76m telescope
- Low Surface Brightness Galaxy
- Low-mass X-ray Binaries
- Lower Meridian
- Luminosity
- Luminosity-Decline Rate Relation