What does 'offline' mean for flyback regulators/controllers?

If you are working on power supplies, you will often see ICs that are labeled offline flyback controllers/regulators (for example, the LT3799).

What does offlinemean in this context?

Offline means that the flyback regulator operates **off the AC line,**i.e. it is intended for use with the 115VAC / 230VAC grid. There are no online flyback controllers, but many other flyback controllers are not suitable for use with a very high (and typically rectified AC) input voltage but can only be used to convert DC/DC

Typically, offline controllers require a bridge rectifier and some additional circuitry like EMI filters to be connected to the AC grid, so be sure to check their datasheet and/or application notes on how exactly they are intended to be used.

Note that designing high-voltage circuits such as offline flyback power supplies can be dangerous and should only be done by experienced engineers.