QNH. The atmospheric pressure adjusted to mean sea level.

Refer to figure.
ICAO Doc 4444
4.10.4 Provision of altimeter setting information
4.10.4.3 The flight crew shall be provided with the transition level in due time prior to reaching it during descent. This may be accomplished by voice communications, ATIS broadcast or data link.

2.4.3 References to vertical positioning after approach clearance
After approach clearance has been issued and the descent to land is begun, the vertical positioning of an aircraft above the transition level may be by reference to altitudes (QNH) provided that level flight above the transition altitude is not indicated or anticipated.

When an aircraft is flying at or above the transition level, the vertical position of the aircraft shall be expressed as a flight level with reference to QNE (1013.2). When an aircraft is descending below the transition level, the vertical position of the aircraft shall be expressed as an altitude with reference to QNH. In order to change from 1013.2 hPa to QNH during descent, the pilot needs to know the transition level, which will be provided by the ATC. A change to QNH is allowed when cleared to descent to an altitude below the transition level and the aircraft is not expected to level off above that altitude.