A space-time fusion, which the Sixth Doctor called a congruence, was a link created between two space-time vehicles.
Steel Specs performed a space-time fusion between his 1934 Citroën Traction Avant and the Doctor's TARDIS in order to gain access to the TARDIS to reach Kolkokron, leaving the Doctor in the Citroën. The fusion was dimensionally unstable because they were incompatible vehicles, and the Doctor broke the congruence by jumping dimensions, which caused the Citroën to lose geospatial coherence. (AUDIO: The Hollows of Time)