Well!  I can tell you some about my locality, the Pacific Northwest, where hardpan is just as abundant as grass.  

My understanding is that this layer consists basically of glacially-derived sediments that have been super-compacted at the base of glaciers that came through here as recently as 13 ka.  I've been on projects where steel excavator teeth were replaced on a regular basis due to the stubbornness of the stuff.  

Anyway, just feeling talkitive.  Why do you ask?

I'm in the Southeast.  Contractors use the term loosely, and I was just wondering  about how many different kinds of soils the term was applied to.  We don't have anything highly consolidated.

There probably is not a "common" definition across the country - or across the world.  To me, hardpan would be very hard/very dense glacial till (clayey or sandy till) - or partly weathered rock/residual soil.  This is one reason to keep such terms out of specifications.  They are good for a "feel" but can cause a lot of problems if in the specifications.  Example, here in India, there is the term moorum - residual lateritic soil; not defined other than most people just "know" what it is.  In specs, stay with defined technical descriptions.

jgailla...hardpan is a colloquial term that has numerous meanings.  In your area (SE Coastal plains), it is generally considered to be a slightly cemented to well cemented, slightly silty to silty fine sand.  It is usually organically stained and exists in strata varying from a few inches to several feet thick.  The cementation is natural, often caused by rising and falling water table, and should not be confused with density.  Just because it's hard, doesn't mean it's dense.

When you encounter it, it should be broken up to allow appropriate compaction of the material.  It can be compacted after breakup.  It is usually moisture sensitive (on the Proctor curve) and when saturated can be problematic on sites because it tends to dry slowly.

The term Hardpan in Colorado includes:
- compacted glacial materials, as described above.
- soil cemented with Calcium Carbonate 'Caliche', due to rising/falling water table.
- soil cemented with Sulfate (gypsum) 'caliche', often due to evaporation from a seasonally high ground water.

From this tread we can see the range of meanings and the admonition from BigH to avoid the term in specifications.

Arizona - hardpan refers to partial to total cementation with carbonate.  Here, it is not due to a falling water table, but to the alkaline nature of the soil.

However, caliche is another term that is used around her, but is not recommended, and for the same reasons.  A better term might be "partially cemented".  

Funny I ran across this thread. Last week in our office this toppic came up.  Two theories came out.
1) Hardpan - hard soil on the surface which was compacted by pans (earth moving equipment)

2) Hardpan - a gold miners term for the hard material at the bottom of a stream wich is hard to dig with a pan

It has been a hard day and basford helped get me through it. I appreciated the smile.