The designated senior leader, required under standard 16/S1, who champions and oversees an organisation's information and records management programme. Not an operational IM manager.
The Executive Sponsor is the senior owner of the IRM programme required by the standard: a leader with organisation-wide influence, not an operational IM manager. Your IM/RM team should be able to name who holds the role.
IAR turns terms like this into working practice: assets classified against your disposal authority, stewarded by position, and ready to evidence.