Direct sun would be outside with no shade. Direct sun would be in a window with light coming through all day, a South facing window, here in the US. What sort of plant had that tag?

I would understand "full sun" to be a location that receives sunlight all day long. Direct would mean unfiltered sunlight. The north side of my house gets direct sun but only part of the day so I think that would be partial and direct sunlight.

No.

It's sort of like asking if there is a difference between 'soil', 'potting soil', 'substrate', and 'medium'. In this case and for the most part, the words are interchangeable and context determines the meaning unless the discussion includes a newcomer to growing things in containers.

'Direct sun' could be outdoors in the middle of a yard with no shade for a furlong, or it can mean sitting in front of a S window (in the N hemisphere), or in a S-facing conservatory - even if light passing through glass typically reduces visible light transmittance by 20-30% for applications utilizing insulating glass units (IGUs), depending on what products are used in their fabrication, and at least 10% for a single lite of SS (single strength 2.5mm/ 3/32") glass.

Many low light plants can tolerate a full sun photo load if they are sited in front of windows containing IGUs. What they often don't tolerate is the passive solar gain (heat build-up) that occurs when the light strikes the leaves. Imagine how hot a dark green car gets in intense direct sun when there
is no breeze to get a sense of what passive solar gain is - light energy
being converted to heat. Overheating commonly occurs indoors because of
a layer of still air (called the boundary layer) that surrounds the
leaf. It serves to insulate the leaf so heat is trapped in the leaf and
not lost to surrounding air. A tip that allows your plants to enjoy more sun if you help reduce heat build-up: Most cases of overheating can be avoided by
rigging a fan on a timer so it disrupts the boundary layer during
periods of intense photo load.

Al

Mentha, the plants that had the tag were a couple of trees in the nursery section of the store. I remember a Pachira being one of them, not sure about the rest.

"Direct sun means the sun is shining directly on the plant or location. Full sun means there is direct sun for 6+ hours per day."

Name one plant that grows best with more than zero but less than 6 hours per day of direct sunlight.

These terms have been around for a very long time and are not arbitrary or subjective, in regard to gardening in the US. In any gardening book or on any website, full sun is defined as 6+ hours of direct sunlight. Direct sunlight is when the sun is shining right on a plant without any shadows.

It's well known that some plants need some shade at lower latitudes, where they might become etiolated at much higher latitudes with the same amount of exposure. So in addition to knowing ones' zone, it's always best to solicit/gather gardening info that's as local as possible. A lesson I learned well after moving from Z5 OH to Z8 AL, which is a very minor change, considering the planet as a whole. Many plants that loved full sun there need some shade here. Whether or not a particular plant needs direct sun, and the amount of it if so, are care factors that definitely hinge on location.

Latitude/intensity and whether the sun is passing through glass are factors that are worthy of consideration but are extraneous to assessing whether or not sun shines directly on a plant, or measuring the duration of that exposure.

Azurinski, there are millions of plants that do not need any direct sunlight, perfectly happy to spend their lives in the shadows.

I think the primary problem here is relying on a plant tag to give complete or even accurate information. Even when the grower tries hard to be conscientious about the info on the tag (which is pretty rare in my experience) the medium is a pretty limited way to convey information.

There's no substitute for experience, although a quick Google on your phone can help fill in gaps in the memory! Always assuming the plant tag is accurate as to what plant is in the pot.

"Name one plant that grows best with more than zero but less than 6 hours per day of direct sunlight."

Feeling naughty ... Tongue in cheek, Fittonia.

"Azurinski, there are millions of plants that do not need any direct sunlight, perfectly happy to spend their lives in the shadows."

You are not addressing anything that I said which demonstrates a remarkable lack of understanding English language considering your alleged distinctions between "direct" and "full" which are not in any dictionary.

"Fittonia"

According to

http://houseplants.about.com/od/More_Plant_Profiles/p/Fittonia-How-To-Grow-Fittonia-Houseplants.htm

Light: Indirect or dappled sunlight.

More generally, every plant grows best when given chronologically consistently intense light for at least 12 hours per day. That is to say, a shade loving plant grows best under consistently low light and a direct sunlight loving plant grows best under consistently high intensity light. Plants dislike having to adapt to varying light intensities. In my experience, most plants can grow better under artificial light than daylight because the light intensity is consistent with time, no cloudy days or changes during the light period.

Thus, the alleged distinction between direct and full sunlight is not only useless but misleading, even if it exists as inbred jargon among some small group of people.