In the context of computer networks, a domain is a set of computers on a network that are managed as a unit.
In the context of the Internet, a domain or domain name is used to identify resources such as websites and email services. It's what comes after "@" in an email address, or after "www." in a web address. For example, in the address https://www.up.edu.ph and email address username@up.edu.ph, the domain name is up.edu.ph.
According to wikipedia.org, a domain name is a string that identifies a realm of administrative autonomy, authority or control within the Internet. Domain names are used in various networking contexts and for application-specific naming and addressing purposes. In general, a domain name identifies a network domain, or an Internet Protocol (IP) resource, such as a personal computer used to access the Internet, or a server computer. Domain names are often used to identify services provided through the Internet, such as websites and email services.