The ambulatory highlighted on a typical cathedral plan
An ambulatory is a set of aisles that extend behind the apse of a church. It might give access to chapels, and it might also be used in processions.
In Rome, the only example of an ambulatory is found in San Carlo al Corso, where the heart of St Charles Borromeus is kept in a reliquary behind the apse.
The term might also be used of other walkways stretching around an area, such as around an atrium.