Apical delta refers to the branching pattern of small accessory canals and minor foraminae seen at the tip or apex of some tooth

Tooth

Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

 roots.
The pattern is said to be reminiscent of a river delta

River delta

A delta is a landform that is formed at the mouth of a river where that river flows into an ocean, sea, estuary, lake, reservoir, flat arid area, or another river. Deltas are formed from the deposition of the sediment carried by the river as the flow leaves the mouth of the river...

 when sectioned and viewed using a microscope

Microscope

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

. Because the anatomy of this area is very small and complex with several portals of entry to the root canal

Root canal

A root canal is the space within the root of a tooth. It is part of a naturally occurring space within a tooth that consists of the pulp chamber , the main canal, and more intricate anatomical branches that may connect the root canals to each other or to the surface of the root.-Root canal anatomy:...

 ie. more than one apical foramen

Apical foramen

In anatomy the apical foramen is the opening at the apex of the root of a tooth, through which the nerve and blood vessels that supply the dental pulp pass...

 this may make successful endodontic treatment less likely.

The root tip is removed during apicoectomy

Apicoectomy

A root end surgery, also known as apicoectomy , is an endodontic surgical procedure whereby a tooth's root tip is removed and a root end cavity is prepared and filled with a biocompatible material....

to eliminate the apical delta and maximise the chance of successful healing.