Ionic column

An Ionic column was a piece of architecture the Doctor's TARDIS disguised itself as, before its chameleon circuit malfunctioned. (TV: An Unearthly Child). The Master used to disguise his TARDIS as a Doric column using his functional chameleon circuit. (TV: Logopolis, Castrovalva, Time-Flight)

Behind the scenes
The shape of the Master's TARDIS on Logopolis is actually a Doric column, contrary to popular belief and its description as "ionic" in The Discontinuity Guide (see DWM 412). Ionic columns are actually much more ornamented. According to the novelisation of Time-Flight, the Master's TARDIS was Corinthian, although it was still the same shape on screen.