What best describes the essence of 'residual'?

The term 'residual' fundamentally refers to what is left over after the main part has been removed or dealt with. In various contexts, such as mathematics, economics, or everyday usage, 'residual' denotes a quantity or element that remains after a process has been completed. For instance, in a mathematical function, residual values represent the difference between observed and predicted values, highlighting the quantities that are not explained by the model.

In contrast, 'complete' suggests a state of fullness or wholeness, which does not align with the concept of something being leftover or remaining. Similarly, 'essential' refers to something that is a fundamental or indispensable part of a whole, while 'integrated' implies a merging or combining of elements into a cohesive unit. Both of these terms do not convey the idea of something that is left over after the primary components are accounted for. Thus, the essence of 'residual' is best captured by the idea of the 'remaining part.'