1. Act of maintaining controlled environmental conditions for the purpose of favoring growth or development of microbial or tissue cultures or to maintain optimal conditions for a chemical or immunologic reaction. 2. Maintenance of an artificial environment for an infant, usually a premature or hypoxic one, by providing proper temperature, humidity, and, usually, oxygen. 3. The development, without sign or symptom, of an infection from the time the infectious agent gains entry until the appearance of the first signs or symptoms.