I don't think I've ever seen a set definition. My take is an option or accessory that was available when the car was manufactured and in general use.

Here's Google's AI definition: 

I will give an example that illustrates my definition of period correct.

I am planning to add turn signals to my 1936 Chevy 1-1/2 ton truck. I would not think of doing so by using a shiny chrome plated switch with a 4-way flasher feature and then buying some lamps at a local store. It is my opinion that turn signals were commonly added to farm trucks when they began to show up as OEM and aftermarket accessories, in the 1950s. To accomplish my conversion, I have located a nifty old switch that is appropriately "toasty", like my truck. Furthermore, I have sourced a pair of lamps for the rear that illuminate an arrow so that vehicles following me will see a flashing arrow, pointing in the direction of my impending turn. For the front, I chose a pair of small lamps with amber glass "beehive" lenses that will mount on the outside of the frame, just above the bumper. Another choice for the front would have been fender mounted lamps, but I am not big on drilling new holes in my truck.

The goal is to have my entire turn signal setup to look like it was installed in the 1950s, therefore being "period correct" for when (during its working life) signals would likely have been added.

Mike

I would have to agree with what has already been said.

Period correct to me means something that would have been available or used when the car was new or still in common use.

Or, as Mike points out, something that was available and commonly used later on during the time when earlier vehicles were being updated.

And, for me, period correct isn't limited to just the accessories available from the manufacturer.