1. "Yes" This is the most common meaning of "si" in Spanish and Italian.
2. "If" This is another common meaning of "si" in Spanish and Italian.
3. "Silicon" This is a chemical element with the symbol Si.
4. "Sir" This is a polite term of address for a man, particularly one in a position of authority, often used in British English.
To know which English word "Si" translates to, you need to consider the context of the sentence or phrase.