A period within the life cycle of an entity that relates to the state of its description or realization. (ISO/IEC/IEEE 15288)
ISO/IEC/IEEE. 2015. Systems and Software Engineering -- System Life Cycle Processes. Geneva, Switzerland: International Organisation for Standardisation / International Electrotechnical Commissions / Institute of Electrical and Electronics Engineers. ISO/IEC/IEEE 15288:2015.
A lifecycle stage is also referred to as a phase.