Is there a difference between "post-medieval" and "modern"?

The short answer is "Yes, but it's complicated".

The problem here is one of definitions, and there are many, overlapping, definitions of archaeological periods in use.

In the context of the United Kingdom, the medieval period ended with the dissolution of the monasteries and the Reformation, so "post-medieval" simply means "after the Reformation". However, the Reformation took place over a number of years, so where should we place the dividing line?

Medieval

The medieval period in England runs from the end of the Romano-British period to the beginning of the early-modern period, so from somewhere in the fifth century to the Reformation.

Putting exact dates to that period can be difficult, but you will commonly find Honorius's rescript to the Civitates of Britannia, dated to AD 410, as a terminus for the Romano-British period.

Establishing an exact date for the start of the Reformation is problematic, as noted above. When I was a student, it was common to choose 1540 as the transition date. However, many archaeologists and historians prefer to opt for the end of the Plantagenet dynasty (at the Battle of Bosworth), which can be firmly dated to 1485, as an end date for the medieval period in England.

So "medieval" means AD410 - 1485 (or perhaps AD410 - 1540, or some other, similar date range, depending on definitions and personal tastes).

Early Modern Period

The "early modern period" in Britain essentially covers the 16th, 17th, and 18th centuries. It terms of dates it runs from the end of the medieval period (i.e. from 1486, as discussed above) to the Treaty of Union between Great Britain and Ireland in 1801, which created the United Kingdom of Great Britain and Ireland.

After 1801, we enter the "late modern" period...

Post Medieval

"Post medieval" is often defined as the period between the medieval and the industrial ages. In terms of dates, it might be defined as running from 1486 to 1901.

Post-medieval archaeology even has its own society, the Society for Post-Medieval Archaeology. Their interests are:

"the archaeology of late medieval to industrial society in Britain, Europe and those countries influenced by European colonialism."

So, in their case, "post-medieval" is defined rather differently! (I'm sure you've detected a theme by now)

Modern

In this context, "Modern" can mean anything in the early-modern or late-modern periods, i.e. anything since the medieval period. So, technically this would include everything from 1485, until yesterday. OK, well not quite yesterday, but you take my point.

To confuse matters still further, you will often hear specialists in Romano-British archaeology referring to anything post-Roman as "modern". This is usually with tongue firmly in cheek, but I have occasionally seen things like shards of Norman pottery in a Roman context referred to as "modern intrusions"!

Of course, as if all that wasn't complicated enough, all these periods can be subdivided into further archaeological periods. What is more the dates for each period are not always universally agreed. As another example, these definitions are used for the archaeomagnetic dating database being developed by Bradford University.

The key thing to remember here is that these are categories for classifying material culture. People didn't suddenly start using different pottery, or buttons, or brooches, etc., just because the medieval period had ended, any more than they did when there was a change of monarch.

It makes no sense to say that a piece of pottery made in 1484 is "medieval", while an identical vessel made by the same potter in the same workshop in 1487 is "post-medieval". Similarly, a bowl made in 1835 may, technically, be "Georgian", while an identical bowl made by the same potter in 1839 may, technically, be "Victorian", but those distinctions would be meaningless. In any case, unless they inscribed the date on the vessel, how would we actually know exactly when it was made?

In cases like these, it would be usual to say something like "late medieval / early post-medieval" or "late Georgian / early Victorian" based on the archaeological contexts in which the pottery had been found (and in the absence of other information that allows more precise dating).

When attempting to understand a particular aspect of a specific period in history on the basis of its material culture, it is often helpful to consider assemblages of artefacts, rather than single artefacts in isolation. So we are often interested in groups of artefacts of a similar type from a broad historical (or prehistoric) period. This is why it is not unusual to see terms like "post-medieval or early modern period" in archaeological reports or databases. The distinctions can be useful when people search the databases by period to identify assemblages. That artefact will show in in searches for both "early modern" and "post-medieval".