Tenkan is a passing movement, turning the body on one foot by 180 degrees.
One foot is the turning point (usually the front foot), so that in the end position the same foot is in front. The tenkan movement starts from the hip, allowing the free foot to make a (big) step.
Very often tenkan is preceded by a direct entry (irimi). The combined irimi-tenkan movement is also known as tai-sabaki, tai-no tenkan or tai-no henko. It is the most common form of passing and stepping out of the line of attack.
The following video shows irimi, tenkan, tai-sabaki and tenkai (= kaiten):