Hi is a common informal greeting used to say hello12. It's more casual than "hello" and is appropriate in most everyday situations when greeting friends, family, colleagues, or acquaintances1.

Some key points about "hi":

• It's widely used as a quick, friendly way to greet someone you know2.
• It can be used on its own or followed by the person's name, like "Hi, John!"2
• It's versatile and can be used in person, on the phone, in text messages, emails, etc.
• While very common in English, similar short greetings exist in many languages.

The word "hi" is believed to have originated in the United States in the 1800s. Its first recorded use is attributed to a Kansas Native American1.

While "hi" is casual, it's not considered rude or overly informal in most contexts. However, in very formal situations (like meeting a dignitary), a more formal greeting may be more appropriate1.