Domain is an online address.
Those who cannot answer to the question “What is Domain?”, should have known what a website and an email are.  Actually, domain is included in those website or email address.
Domain is a string of characters made by some rules, and indicate the specific computer online.
IP address is to specify computers, and when it associates with domain, people will be able to access it without memorizing the address.
Domain is usually reffered as “online address”, but more correctlｙ it is “something that makes online address clearer”.  You can choose the string of characters, but basically it is on a first-come, first-served basis.
Today, a lot of companies use the specific domain name when they launch new product.