Transverse deflection question

Transverse deflection/vertical sway/horizontal deflection- which one is it or are all three correct?
I've discussed this phenomenon with several engineers and they all seem to have different definitions.
I'm talking about when you are looking face on at a truss or bridge, arch in particular, and you have a wind of traffic load on one side of the structure and uplift on the other side.
I have searched but cannot find any real information on the subject... I remember a documentary on tv a long time ago when an engineer stated that there is nothing we can do about it and that by simply adding more diagonal members, which control this deflection, to a two chord truss, will not solve the problem and is the reason why bridges are so limited... the further you span the worse it gets.

Does anyone here have any links on the subject or comments?

Thanks,
Dave