A nexus is a central link or connection. If you happen to be at the nexus of something, you are right in the middle of it, like standing in the middle of an intersection.
Nexus entered English during the seventeenth century from the Latin word nectere, meaning "to bind or tie." People tend to use this word to describe the point where different things or ideas come together or intersect. In the field of cell biology, a nexus refers to "a specialized area of the cell membrane involved in intercellular communication and adhesion," and implies that the nexus of a cell facilitates communication among the various parts and allows it to work properly.