The word endogenous means "arising from within", the opposite of exogenous.
An emotion or behavior is endogenous if it is spontaneously generated from an individual's internal state. So for example endogenous depression is thought to be caused by biological changes in neurochemistry as opposed to reactive depression caused by factors in the environment such as family disagreements.
Endogenous substances are those that originate from within an organism, tissue, or cell . Endogenous retrovirus are caused by ancient infections of germ cells in humans, mammals and other vertebrates. Their proviruses remain in the genome and are passed on to the next generation.
Endogenous processes include circadian rhythms.