"You called this a painting at one point and a sketch at another. What is the difference between a sketch and a painting? This is a discussion my husband and I have fairly often:-)"
To me, a "sketch" tends to suggests a momentary glance or quick impression, executed freely and intuitively, without much thought to how it lays on the page. Sometimes a sketch is a means to an end, a planning stage or a first effort.
I also use "sketch" as a noun or a verb referring to any picture done purely from observation, regardless of media or level of detail. I like the sound of "I'm going sketching" better than "I'm going plein-air painting." Sometimes I sketch indoors, and "indoor plein-air painting" is a contradiction in terms.
A "painting" might have more consideration behind it, perhaps more thought of composition or overall effect. A painting might demand the use of better materials— a true sketch might be something you could draw on a shopping bag in the heat of the moment.
I like the term "study" too, because it gets me into the frame of mind of patiently observing the world as it is. My goal when I'm working on location is often to try to portray something unfiltered by preconceptions. Not easy! Being patient and open enough to find a fresh motif, and then to try to see beyond artistic conventions, is the most difficult challenge for me in observational work, but it's also the most productive mindset to cultivate.
By the way, I have a hard time distinguishing between the terms "drawing" and "painting," especially when using water-soluble colored pencils, which can be used wet or dry in the same picture. Art historians often group watercolors with drawings, even if they're made entirely with a brush. I suppose that has a lot to do with the way watercolors are stored and displayed in museums.
Previously on GJ: Why do we have no word in English for a person who draws?