Zen is the seventh letter of the Samaritan alphabet and carries the numeral value seven. It writes a plain z. The name is the inherited Northwest Semitic one behind Hebrew zayin, and the gloss usually attached to it, a weapon or a sword, is a reconstruction from the word rather than an attested meaning.
The letter is a good place to see what an alphabet of twenty-two signs had to leave out. The ancestor language behind Hebrew, Aramaic, Samaritan and Arabic had more consonants than the alphabet has letters, and when the signs were fixed, several distinct sounds were assigned to a single letter. Zen is one of those: it carries both the z of the older language and a second sound that Arabic still writes with a letter of its own. Speakers of the languages that used the twenty-two letter alphabet had already merged the two, so nothing was lost for them, but the merger is why an Arabic word and its Hebrew or Samaritan cousin sometimes look further apart than they are.
The consequence for a reader is small and for a comparative linguist it is large. It means the alphabet is a record of one stage of one branch of the family rather than a neutral inventory of Semitic sound, and that a letter can be doing more work in the history of the language than it appears to be doing on the page.
Greek took the sign as zeta, keeping the seventh position. The character is encoded at U+0806.