Meaning of "optical" in "Optical cap, 8X Strip" (used in qPCR)
Does the word indicate the the cap is fully transparent? Or that the cap creates no visual distortion (it is flat)?

Yes and yes. As an example of the different kinds of PCR tube caps, refer to the cap options available from Thomas Scientific:

Available with a domed, flat frosted, or flat clear caps for qPCR. [emphasis mine]

For most lab plastics, &quot;optical&quot; refers to the surface between the camera/detector and the sample.  Another example: for plate-based fluorescence assays where the plate reader takes measurements from below  the plate, one might use a Nunc MicroWell 96-Well Optical-Bottom Plate from ThermoFisher. Usually, such plates also have opaque walls to prevent signal bleed from adjacent wells.