observable
An observable is any physical quantity that you can actually measure — the position of a particle, its momentum, its energy, the component of its spin along some axis. In quantum mechanics each observable is represented by a Hermitian operator, and the link between the abstract operator and a number on a dial is the heart of how the theory makes predictions.
When you measure an observable, the only results you can ever obtain are the eigenvalues of its operator. Before the measurement the state may be a superposition that has no single definite value; the act of measuring forces one of the allowed eigenvalues to appear, with a probability fixed by the state. This is why a quantity that looks perfectly ordinary, like energy, can come in discrete steps for a bound system.
Not every operator is an observable. To qualify, an operator must be Hermitian, which guarantees that its eigenvalues — the possible measurement outcomes — are real numbers, as any honest meter reading must be. Two observables can be measured together to sharp precision only when their operators commute; otherwise nature forbids them both from having definite values at once.
Every measurement of an observable yields one of its operator's eigenvalues — nothing in between.
Calling a quantity an observable does not mean it always has a value waiting to be read. Until measured, an observable can be genuinely indefinite, not merely unknown — a distinction at the core of quantum theory.