Meaning of Native plant
What are Native plants? Does the phrase refer to state (native Maryland plants) or region (native Mid-Atlantic)?
Howard County Maryland
The definition of "native" has many meanings depending on who is asked. For many organizations and plant societies, the species deemed native have been documented as growing in this area since pre-colonial times. (In other words, before most human influence, though the impacts Native Americans had on plant communities may still be at least somewhat unknown.) What "this area" means also depends on the group in question; usually modern state lines are used (here there is a Maryland Native Plant Society), but this may be more for convenience in categorization since plants obviously don't observe such boundaries. (For states that have major geological features as their state boundaries, this delineation is more apt.) Maryland shares the vast majority of its native flora with our neighboring states, so any regional mid-Atlantic reference list is likely just fine to use here, especially as it relates to gardening.
Plants occur in swaths of habitat, so a species native to MD won't necessarily be considered native to all of the state. For instance, a plant that specializes in growing in a coastal plain habitat will not be native to the habitat of our western-county mountains. Some nurseries and gardening guides use "native" more broadly, referring to a region rather than a particular state. This is largely for reasons of practicality because they publish materials or sell plants to multiple states. This broader use of the term can also make sense because the same habitat will span multiple states. (To continue with the example above, the Delmarva peninsula technically comprises three states, but largely the same coastal plain habitat. Therefore, using state boundaries, one could say a certain species there is native to all three states, even though it may only be found on that peninsula. With regards to Virginia, this would mean that while this hypothetical plant occurs in a tiny fraction of that state overall, a state-wide native plant list would still include it.)
As another example, Oakleaf Hydrangea is sometimes referred to as "native" when discussed or marketed in our area when it does not actually occur in MD in the wild; instead, its true native range lies to our south. An even starker example would be Mahonia aquifolium, a species native to the Pacific northwest (and suggested by some gardeners as a "native" alternative to replace popular Asian Mahonia species which are showing somewhat invasive tendencies in mid-Atlantic landscapes). As a North American (or U.S.) native species, Mahonia aquifolium is sometimes questionably lumped-into the native gardening suggestions for our area even though it's natural range is nowhere near here. In this case, "native" is being used as a continent- or country-wide label.
Here is an example of a Maryland-specific checklist published by the Maryland Native Plant Society: https://mdflora.org/Resources/Publications/SurveyData/mdchecklist.pdf. Because of the length of these all-inclusive lists, plants are grouped alphabetically by family after they are first separated into more major groups (ferns, grasses, conifers, etc.).
Native plant lists for gardening purposes will be much shorter than the example above (but still diverse, numerous, and useful) because they tend to skip-over species that don't offer much garden aesthetic (some are even considered weeds in gardens). Plants that even native nurseries don't offer tend not to be included because there will be no practical way to obtain them for garden use. The species range that nurseries grow is ever-expanding, though, as gardening with native plants becomes more and more popular and the nurseries learn how to propagate challenging species more successfully.