Pletho (πλήθω) means to fill or to be full. Closely related to pleroo (G4137) and pimplemi. In the NT it is often used in the passive — being filled — particularly with the Holy Spirit, emotions, or spiritual qualities.
The most significant use is in connection with the Holy Spirit. At Pentecost, disciples were filled with the Spirit (Acts 2:4). Peter was filled before addressing the Sanhedrin (Acts 4:8). Paul was filled at conversion (Acts 9:17). Elizabeth was filled upon Mary's visit (Luke 1:41). This 'filling' represents the Spirit's empowering presence for specific tasks — bold preaching, prophetic utterance, faithful witness. Unlike permanent indwelling at conversion, the filling is repeated, dynamic empowerment. The pattern in Acts: every great gospel advance was preceded by Spirit-filling.