Wiktionary
vb. 1 (context idiomatic English) To enter service or become active. 2 (context idiomatic English) To login to an internet communication system, such as an instant messenger, online game or forum.
Usage examples of "come online".
The first organizations to be linked via the Net - universities - had far poorer security than the government agencies and corporations that had more recently come online.
I rouse once more, and additional circuits come online as I realize this is not a regularly scheduled Alert cycle.
It would be a race against time to determine if these facilities could come online quickly enough to replace America’.
The evening maquiladora shift was still hard at work, laboring in the vast spread of manufacturing and assembly plants and their attendant facilities, and the bulk of the night shift wouldn't come online for another hour yet.