- Rayleigh-Ritz method
- Rayleigh scattering
- Rayleigh-Tayler instability
- ray of a congruence
- Razumikhin's condition
- R-chain
- r-closed space
- R-conjugacy
- R-controllability of singular system
- r-dimensional skeleton of simplicial complex
- reachability
- reachability of Petri net
- reachable set of singular system
- reaching law
- reaction-diffusion equation
- reaction-diffusion equations system
- reaction-diffusion logistic equation
- totally convex set
- totally defectless valuation
- totally defectless valuation ring
- totally disconnected space
- totally geodesic map
- totally geodesic submanifold
- totally geodesic submanifold of symmetric spaces
- totally inaccessible time