What is the X in X-mas?
1711, "act of joining," from Latin iunctionem (nominative iunctio), noun of action from past participle stem of iungere "to join together" (see jugular). Meaning "place where things meet" first attested 1836, American English, originally in reference to railroad tracks.
junction junc·tion (jŭngk'shən)
The act or process of joining or the condition of being joined.
A place where two things join or meet, especially a place where two things come together and one terminates.
A transition layer or boundary between two different materials or between physically different regions in a single material.