What do you call an instrument that can help a pilot fly along a predetermined flight path (e.g. path thru GPS coordinates), showing them when they stray a little so they can correct? Not a control system, but just a visual feedback display. I want to say I've seen something like it before, that had two panels each with a plane symbol and "reticle", but I would like to know what it is that I'm describing. Thanks
What you are describing is a Course Deviation Indicator or CDI.
These are fairly standard across most aircraft however they may also be combined with a heading indicator resulting in an Horizontal Situation Indicator. There are also variants that have an aircraft symbol such as you describe.
A lot of GPS units with moving maps are generally able to display the info you describe as well.
There are even various apps that allow you to display such info on your tablet or smart phone