An operator is a rule by means of which is given function is changed into another function.
The measurable quantities like energy, momentum, position etc. are called observable. Each observable has a definite operator associated with it.
Operators Associated with Dynamical Variables
9.7.1 Energy Operator
The Schrodinger equation
HΨ = EΨ
informs, that the operator associates with energy E is Hamiltonian H, i.e., Eop or E – H
The time independent form of Eop is