A more thorough explanation:

Definition: Litera (also spelled littera) is a Latin word that means "letter." It has two historical meanings:

1. A single letter of the alphabet.
2. The literal interpretation of a law, as opposed to its intended meaning.

When we use the word "letter" in English, we usually mean a written message that is put in an envelope, sealed, and delivered. This is a different meaning of the word.

Examples:

• "The word 'cat' has three letters."
• "The judge ruled that the defendant's actions violated the letter of the law, even if they did not violate the spirit of the law."

The first example uses "letter" in its usual sense, to refer to a single character in the alphabet. The second example uses "letter of the law" to mean the exact wording of a law, without considering its intended purpose or spirit.