In networking, WAN stands for Wide Area Network, which refers to a network that spans a large geographical area, such as cities, countries, or even continents. It connects smaller networks like LANs (Local Area Networks) and MANs (Metropolitan Area Networks) for global communication.

Octal, Hexadecimal, and Binary are types of number systems used in computing to represent data. Boot, on the other hand, refers to the process of starting up a computer system and is not a number system.

A valid email ID must have the format username@domain.com. The option xyz.yahoo.com is invalid because it does not include the "@" symbol, which is required to separate the username and the domain in an email address.

A compiler is a software that translates programs written in a high-level programming language into a low-level language, such as machine code, so that it can be executed by a computer. It performs the translation in a single process and ensures the program runs efficiently.

Java is a high-level, object-oriented programming language widely used for developing web applications, mobile apps, enterprise software, and more. It is not a hardware, storage device, or bootable device.