A more thorough explanation:

Definition: Littera is a Latin word that means "letter." In history, it refers to a letter or the letter of a law, as distinguished from its spirit.

Examples:

• When I received the littera from my friend, I was excited to read what she had written.
• The litterae of the law were clear, but the spirit of the law was open to interpretation.

The first example illustrates the use of littera to mean a letter, as in a written message. The second example shows how littera can refer to the letter of a law, which is the literal text of the law, as opposed to its intended meaning or interpretation.