Why is the thalamic reticular nucleus called a nucleus?
The thalamic reticular nucleus (TRN) is a thin layer of GABAergic cells adjacent to the thalamus (Fig. 1).  Because of its strategic location between the thalamus and cortex, the TRN is often suggested to be important to attention and regulation of information flow between the thalamus and cortex (Lam &amp; Sherman, 2011).

A nucleus is defined as a


  [p]ortion of tissue that are compact accumulations of neurons having roughly similar connections and functions


Reticular is defined as 


  A fine network or net-like structure.


So reticular nucleus translates itself as a net-like accumulation of neurons.

In contrast to the wikipedia meaning, therefore, I interpret nucleus as more of a cluster of functional organizational units rather than a morphological characteristic of that cluster per se.  


Fig. 1. Thalamic reticular nucleus. source: Dana Foundation

Reference
- Lam &amp; Sherman, J Neurosci (2011); 31(18): 6791-9