Hi Paul,
Off the top of my head, I don't know of an overall glossary although one would certainly be useful. Digital Photography Review does have a glossary of some terms that are commonly used in reviewing digital cameras, so that might be of some help. Here's my sense of the meanings of the terms you mention:
1. Upressed, or more commonly ressed up, means a digital image file that has had its resolution increased. See #2. "Resolution" in this context usually just means the number of pixels, which may be something of a misnomer. See below.
2. Interpolate up or down, means applying a mathematical algorithm to a digital image file to proportionately increase or decrease the number of pixels, and hence the resolution of the file, by means of a process of interpolation. Interpolation is a general term that describes a process of estimating intermediate values in a pair or series of values based on the known values. For example, if a file has a series of pixels with values 20, 40, 60 and you ress up the file so that that series has double the number of pixels, the resulting series would be 20, 30, 40, 50, 60, 70. Technically, the process of estimating the final value is extrapolation since the value lies outside the known series (and of course the additional value could also appear at the beginning rather than the end). Any time you change a file from its native resolution, you necessarily must estimate some of the resulting values.
3. Native resolution--the default size of the image file produced by a digital camera or scanner, which may be expressed as a total number of pixels, e.g., 6 megapixels, or as pixel dimensions, 2000 by 3000 pixels. The native resolution very closely corresponds to the number of photo sites on the camera's sensor (usually a few less) and does not count any increase or decrease by interpolating up or down performed in camera (such as so-called digital zoom). (To make matters more confusing, you should know that most cameras apply an interpolation type routine known as de-mosaicing to arrive at the native resolution file, but the resulting number of pixels still corresponds to the number of photo sites on the camera's sensor.)
So, to use all the terms, a file with a native resolution of 2000 by 3000 pixels might be ressed up by interpolation to a final size of 4000 by 6000 pixels.
One of the confusing things about the concept of "resolution" is that it can mean slightly different things in different contexts. When the term is used in reference to the digital file, it simply refers to numbers of pixels without any dimension. But when we describe the resolution of a camera system, we are describing its ability to resolve details in the scene being photographed, e.g., lines per millimeter. When we use resolution in reference to printing or other output methods, dimension is also an issue, e.g., one might print a file at a resolution of 240 pixels per inch. So, a file at its native resolution of 2000 by 3000 pixels could, without interpolation, be printed at different output resolutions--20 by 30 inches at 100 pixels per inch or 10 by 15 at 200 pixels per inch, etc. Obviously, a bigger (higher resolution) file will typically be better able to resolve details than a smaller one, other things being equal, and will be able to produce a higher resolution output.
Hope this helps.
