reliability
is σ2/n where σ2 = Var (X), so, as the sample size increases, 
becomes a better estimator in the sense that the variability decreases.
- real projective plane
- real world
- rearrangement
- reciprocal
- rectangle
- rectangular
- rectangular distribution
- rectangular hyperbola
- rectangular number
- rectilinear motion
- recurrence relation
- recurrent
- recurring decimal
- recursion theory
- reduce
- reduced echelon form
- reduced set of residues
- reductio ad absurdum
- reduction formula
- redundancy
- redundant
- re-entrant
- reflection
- reflex angle
- reflexive relation