We use this of a person or company whose work is done on the Internet, or whose whole business is the Internet. The name comes from the ending of many website addresses: a little dot, then “com” (short for commercial). A “dot-com company” sells online, runs a website as its shop, or lives on the web in some other way. People also shorten it to “a dot-com”, meaning the company itself. Around the year 2000 a great many of these companies grew very fast and then a lot of them fell apart; newspapers still call that time the dot-com boom or the dot-com bubble. The word can describe the company, the years, or just the idea that the business is online. It is a little old now: younger people often say “tech company” or “online business”, though actually older speakers and business pages still like “dot-com”. It is everyday business talk, not school-formal, and it does not mean every firm that happens to have a website.