I literally not was sure what people meant by "linear." Now as it seems that they do not mean linear by "linear" I better understand why people think that "linear" adventures ae different thing than railroading! But yeah, it is a terrible word choice that is bound to lead to confusion, so on those grounds I certainly object it!
It's actually pretty straightforward.
Linear means One(ish) direction to go. An adventure is linear when the PCs only have 1 specific path from start to completion (there may be branches on the path, even some divergent options but it still goes to the same place). So as has been said A-B-C-D.
But this fact is made clear to the players. They understand that they need to go to place A then B then C all the way to Z to complete the adventure. The players understand they can go a different way, but that will lead them to a different adventure or even something improvised or even the DM saying "hey guys, you asked me to run this adventure - it's what I have prepared today."
The players choices are certainly constrained, but they are making meaningful choices based on those constraints - even if the meaningful choice is to follow along rather than to abandon the advenutre.
Put another way, the players are following along the path because they want to, they have bought in.
Railroading (to me, and the negative connotation I and others have always attached to it) is when the players are denied meaningful choice, even when the only meaningful choice is to buy in to the linear plot. The players are told they have options 1, 2 and 3 but when they try 2 or 3 their path is blocked or the DM substitutes 1 without telling them (illusionism). This generally involves force or deception, and the players have NOT bought in, they do not want to be on this path - but the DM is pushing them along anyway.
Or put another way, Railroading is a linear path where the DM is hiding (by whatever method) that the path is linear. Linear (IMO) is ok. Hiding that the path is linear, is not.