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Back when I was in college for the fine arts, my instructors and peers used to use a general term for all surfaces an artist would paint or draw on. I recently became active on the Arts & Crafts stack exchange, and suddenly discovered that, having not spoken about art in a technical way for many years, I have forgotten this word. As a result, several of my answers there have very awkward phrasing to skirt the subject. The diversion is intrusive and reduces the quality of my answers, and I worry it may cause them to be misleading.

A similar term would be "matrix" in printmaking, which refers to any block, plate, stamp, etc., which will be altered to create the final image. It covers a wide variety of materials and techniques to refer to them all collectively by their common purpose. It does not, however, refer to the images they make or the surfaces they mark. (The image is a print, the surface is... the word I forgot)

Since I graduated, the college I went to has replaced all of their arts instructors, so I don't really have any contacts there any more. The one class mate I'm still in contact with says he never bothered with that anyways because he didn't want to be a draftsman or painter, so it didn't apply to his goals. None of my textbooks go into that degree of depth in technical language. Only one book has a glossary, and it is aimed at a much younger audience, defining things such as "pencil".

My first instinct is the word medium, which is a catchall. Alternatively, the word support is more specific.

It's important to realize that while "support" is a closer fit, it's rather obscure. For a general audience (and I think A&C counts as general), I would use the word medium.

Here's a quote that goes into some of the technicalities:

In most collections information systems, including The Museum System (TMS) by Gallery Systems, a single primary field—medium—is used to record the physical or material aspects of an artwork. This may include design media (e.g., watercolor, acrylic, gold leaf), techniques and processes (e.g., collage, etching), sometimes the support (e.g., paper, board, other), and often implements and manipulations of media (pen and ink, watercolor with scraping).

_{Descriptive Terminology for Works of Art on Paper}

Support?

The surface, or material, on which an artist creates two-dimensional art. Can be canvas, paper, cardboard or wood panel. The surface often has to be treated before the paint is applied so as to neutralize any natural acidities and protect the work from discolouration or deterioration.

Oil paintings can be painted on a variety of surfaces (also called supports). These can be canvas, panels, paper, wood, metal, plus many others. The reason they are sometimes called supports is because the surface “supports” the medium the image is painted with.

Rice Paper: 

A generic term for Japanese and other asian forms of paper made for artist’s use. Used for sumi-e, brush calligraphy, and watercolor. Fibers from the inner bark of woody plants such as kozo (mulberry), mitsumata, and gampi, and the outer layer of herbaceous plants such as flax, hemp, and jute, are used in manufacturing wide varieties of rice paper.

I’m going to guess you are looking for “substrate.” Canvas, Wood, panel, etc. are surfaces you are going to paint on/create your work on.