letter
In Italian, 'lettera' is used to refer to a written message sent to someone, similar to the English word 'letter'. It can also refer to a character of the alphabet, like the English 'letter'. The context usually makes the meaning clear.
Write a letter to your grandfather.
This sentence is instructing someone to write a letter with 'tuo nonno' identifying the person who should be the recipient.
The letter is on the table.
In this sentence, 'La lettera è sulla tavola' communicates where a specific letter can be found.