Final answer:
Essential nutrients are substances that the body needs for proper functioning and cannot synthesize in sufficient amounts. Nutrients that are not typically needed except at special times are termed conditionally essential.
Step-by-step explanation:
Essential nutrients are substances that the body needs for proper functioning but cannot synthesize in sufficient amounts and therefore must be obtained from food. Nutrients that are not typically needed except at special times are termed conditionally essential. These nutrients are not normally required for daily functioning, but become essential under certain conditions such as illness or pregnancy.