The Zeiss Distagon T* 18mm f/3.5 is the widest prime lens of the Zeiss Z-series.
According to the Zeiss naming convention “Distagon” refers to a “retro-focus” lens based on an asymmetrical design. The asymmetry is necessary to provide the required space for a SLR mirror. The first Distagon lens was introduced in 1953 – a 60mm f/5.6 for the Hasselblad 1000F. The design evolved over time, of course, and it’s no longer just used by Zeiss but across the whole industry.