- essential right ideal
- essential self-adjointness
- essential singularity
- essential singular kernel
- essential singular point
- essential space
- essential spectrum
- essential sphere
- essential state
- essential strategy
- essential submodule
- essential subsequence
- essential superior limit
- essential support
- essential supremum
- essential surface
- essential torus
- essential undecidability
- essential valuation
- essential value
- essential variable
- essential zero
- ess.inf
- ess.sup
- Estes, W.K.