Sen. is the written short form of Senator, used before a name, especially in American newspapers and in Congress: a member of the Senate. You write it with a capital S and, in American style, a period. When you read it aloud, you say the full word “Senator”; you do not say “Sen.” Other countries with a senate — Australia, for example — use the same short form in print. Though actually, sen without the capital can be other things in other places, such as a small coin, and Sen. is not Sr. (senior, father and son) or St. (saint or street). This entry is only the politician. The word feels official and newspaper-like. Friends do not abbreviate each other this way.