China 
Africa 
- Ruddlesden-Popper phase
- Ruderal
- Ruderal species
- Ruggedized
- Ruins
- Rule 184
- Rule induction
- Rule of inference
- Rule of mixtures
- Rule-based machine learning
- Rule-based machine translation
- Rule-based system
- Ruled surface
- Rumble strips
- Rumen
- Ruminococcus
- Run chart
- Run charts
- Run-and-tumble motion
- Run-of-the-river
- Run-of-the-river hydroelectricity
- Run-on
- Run-out
- Run-time infrastructure
- Runaway electrons
- Runge's phenomenon
- Runoff
- Runoff curve number
- Runoff model
- Runtime library
- Runway
- Runway excursion
- Runway incursions
- Runway safety
- Runx1
- Runx2
- Rupture
- Rupture disc
- Rupture of membranes
- Rural health
- Rushton turbine
- Russian iron
- Rust remover
- Rusticles
- Rut
- Ruthenium
- Ruthenium oxide
- Ruthenium trichloride
- Ruthenium(iv) oxide
- Ruthenium-106
- Ruthenocene
- Rutherford backscattering
- Rutherford backscattering spectrometry
- Rutherford scattering
- Rutherfordium
- Rutile
- Rutin
- Ruts
- Ryanodine
- Ryanodine receptor
- Ryanodine receptors
- Rydberg constant
- Rydberg formula
- Rydberg states
- Ryr2
- S transform
- S waves
- S-II
- S-PRISM
- S-Plus
- S-box
- S-expressions
- S-factor
- S-i
- S-layer
- S-nitrosothiol
- S-nitrosylation
- S-process
- S-unit
- S100
- S100 protein
- S1P
- S3
- S9 fraction
- SAA
- SAE J1772
- SAE J2954
- SAE J300
- SAF
- SAF 2507
- SAGD
- SAICM
- SAIFI
- SAM
- SAML
- SAN
- SANS
- SAP ERP
- SAP NetWeaver
- SAR