A turbocharger, comprising: a turbine housing and a turbine wheel mounted in the turbine housing and connected to a rotatable shaft for rotation therewith, the turbine housing receiving exhaust gas and supplying the exhaust gas to the turbine wheel; a centrifugal compressor assembly comprising a compressor housing and a compressor wheel mounted in the compressor housing and connected to the rotatable shaft for rotation therewith, the compressor wheel having blades and defining an inducer portion, the compressor housing defining an air inlet for leading air generally axially into the compressor wheel, the compressor housing further defining a volute for receiving compressed air discharged generally radially outwardly from the compressor wheel, the air inlet having an inner surface that extends for an axial length along a downstream direction, followed by a shroud surface that is adjacent to outer tips of the blades of the compressor wheel; and a compressor inlet-adjustment mechanism disposed in the air inlet of the compressor housing and movable between an open position and a closed position, the inlet-adjustment mechanism comprising a variable-geometry orifice, the variable-geometry orifice in the closed position having an inner diameter (d or) that is smaller than an inner diameter of the shroud surface of the compressor housing at the inducer portion of the compressor wheel, and the variable-geometry orifice in the closed position being positioned such that the effective diameter of the air inlet at the inducer portion is determined by the inner diameter of the variable-geometry orifice, the variable-geometry orifice in the open position being moved such that an effective diameter of the air inlet at the inducer portion is determined by the shroud surface, the inlet-adjustment mechanism further comprising a rotatable actuator member engaged with the variable-geometry orifice for moving the variable-geometry orifice between the open and closed positions, characterised in that the variable-geometry orifice comprises a first annular ring formed in two semi-annular halves (120a, 120b) that are pivotable about a common axis extending transversely to the rotation axis of the compressor wheel, in the closed position the two halves of the first annular ring being substantially co-planar so as to form the first annular ring, in the open position the two halves being pivoted in opposite directions so that the halves no longer form the first annular ring. a turbine housing and a turbine wheel mounted in the turbine housing and connected to a rotatable shaft for rotation therewith, the turbine housing receiving exhaust gas and supplying the exhaust gas to the turbine wheel; a centrifugal compressor assembly comprising a compressor housing and a compressor wheel mounted in the compressor housing and connected to the rotatable shaft for rotation therewith, the compressor wheel having blades and defining an inducer portion, the compressor housing defining an air inlet for leading air generally axially into the compressor wheel, the compressor housing further defining a volute for receiving compressed air discharged generally radially outwardly from the compressor wheel, the air inlet having an inner surface that extends for an axial length along a downstream direction, followed by a shroud surface that is adjacent to outer tips of the blades of the compressor wheel; and a compressor inlet-adjustment mechanism disposed in the air inlet of the compressor housing and movable between an open position and a closed position, the inlet-adjustment mechanism comprising a variable-geometry orifice, the variable-geometry orifice in the closed position having an inner diameter (d or) that is smaller than an inner diameter of the shroud surface of the compressor housing at the inducer portion of the compressor wheel, and the variable-geometry orifice in the closed position being positioned such that the effective diameter of the air inlet at the inducer portion is determined by the inner diameter of the variable-geometry orifice, the variable-geometry orifice in the open position being moved such that an effective diameter of the air inlet at the inducer portion is determined by the shroud surface, the inlet-adjustment mechanism further comprising a rotatable actuator member engaged with the variable-geometry orifice for moving the variable-geometry orifice between the open and closed positions, characterised in that the variable-geometry orifice comprises a first annular ring formed in two semi-annular halves (120a, 120b) that are pivotable about a common axis extending transversely to the rotation axis of the compressor wheel, in the closed position the two halves of the first annular ring being substantially co-planar so as to form the first annular ring, in the open position the two halves being pivoted in opposite directions so that the halves no longer form the first annular ring.