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 offline mean 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.