Theoretically perfect; usually used to refer to a “pure” version of a component that behaves simply. For instance, in electronics, an “ideal” capacitor only has a known and exact capacitance and can be simply modeled mathematically. However, in real life, there is no such thing as a “pure” capacitor; components with capacitance also have resistance, inductance, etc. and are subject to manufacturing error, sensitivity to temperature fluctuations, and so on.

Sourced from Wikipedia.