Online means connected to the internet. Go online, an online shop, online banking.
It works as an adjective and an adverb — an online course, book it online — and is written as one word. It also means available on the internet rather than in person: online learning, online only. Offline is the opposite and also means away from a screen entirely. Note that come online has a separate technical sense meaning to start operating: the new reactor came online in March.
1adjectiveonlineConnected to the internet.
“Half the class could not get online for the first week.”
2adjectivedoneDone or available on the internet.
“The whole course is online and nobody meets anybody.”
online is a state of connection.
The word comes from telegraphy and early computing, where a device on the line was connected to the central system. The line was literal, and the word survived the wires it was named after.
1950, from on + line.
Etymology from Wiktionary, CC BY-SA