* Informal: "Guy" is a casual and friendly way to refer to a man, often used in everyday conversation. It's generally considered acceptable in most social settings.
* Not Slang: Slang usually refers to words or phrases that are specific to a particular group or time period and might be considered more informal or even offensive. While "guy" can be used informally, it's widely understood and accepted.
In Conclusion: "Guy" is a very common and acceptable word, but it's best to use it in informal situations rather than formal settings like business presentations or academic papers.