There are not too many words or concepts in the study of linguistics that begin with the letter "X"; thus I am unable to present a brief list to go along with the current series of definitions. Without a doubt, the most important to theoretical linguistics is "X-Bar Theory".
The above x-bar structure images are from the Linguistics Department at Bucknell University. There are also online lecture notes about syntactic structure at this Bucknell link. These notes provide a brief but clear description of heads, specifiers, complements and adjuncts.