Portfolio management could best be described as:

the co-ordinated management of related projects, including related business-as-usual activities.

the formal process through which changes to the project plan are approved and introduced.

the identification of the benefits (of A. project or programme) at an organisational level and the tracking and realisation of those benefits.

the selection and management of all of an organisation's projects, programmes and related business-as-usual activities, taking into account resource constraints.