In an electric circuit, a linear element is an electrical element with a linear relationship between current and [output[voltage]]. Resistors are the most common example of a linear element; other examples include capacitors, inductors, and transformers.
Fundamentally nonlinear devices like transistors are often used to build approximately linear circuits. For example, an op-amp is designed to behave like a linear amplifier, as long as its input voltages remain within certain limits.