What do you call a protocol prefix?

There's no single, universally accepted term for a protocol prefix. However, here are some common ways to refer to it:

* Protocol identifier: This is the most general term and accurately describes its function.

* Protocol scheme: This is another commonly used term, especially when referring to URL schemes like `http` or `https`.

* Protocol specifier: This is less common but can be used to emphasize the specific nature of the protocol.

* Protocol indicator: This term is less frequent but can be used to highlight that the prefix indicates the type of protocol.

Ultimately, the best term to use depends on the context and the specific protocol you're referring to.

For example, if you're discussing the `http` protocol in the context of URLs, you might use "protocol scheme" or "protocol identifier". If you're discussing the `ftp` protocol in the context of data transfer, you might use "protocol specifier" or "protocol indicator".