An optically correct visor means there’s very little visual distortion when you look through it. Most visors are pretty good to be honest – I don’t think many of us are too troubled by distortion when we’re riding. But if you look carefully, you can probably make out a little wobble or two when you move your head about. With an optically correct visor, there shouldn’t be a wobble at all – everything should look the same whether your visor’s up or down. Here’s more information about optically correct visors.
On this page, you’ll find motorcycle helmets that are advertised as coming with an optically-correct visor (or have one available for it).