Parasitics

From HvWiki

Unwanted circuit components (for example, capacitors or resistors) present in a design. For example a simple wire has parasitic reistance, capacitance and inductance that must be taken into account in some circuits.

When doing simulations it is important to understand parasitics to get accurate and in some cases any results at all. When making high frequency circuits, parasitics can become just as important as the rest of the circuit.