The first problem is what constitutes a medium. The term "multimedia" refers to modalities - sound, images and text - rather than media in the usual sense. It was borrowed off the peg to denote products containing material that hitherto would have been associated with separate media, such as text combined with video clips. Its one significant attraction lies in the fact that it is a plural noun with an irresistible tendency to be used as a singular one. Anybody who works in newspapers knows that there is no greater pleasure one can give than to provide a pedant with a point of order.
Expressions like "electronic media" or "digital media" are hopelessly general. It's somewhat laughable to invest "electronic" with an air of currency, since the world gained its first electronic medium with the advent of valve radio. The encoding of information in digital form is undoubtedly a revolutionary development in communication, but the whole point is that any and every kind of information, from music to pictures, can be rendered into zeros and ones. The issues raised by the introduction of compact discs for recorded music, or the forthcoming experiments in digital broadcasting, are primarily technical and commercial. A symphony or a film are the same whether they are delivered in analogue or digital form.
We need a term which expresses what is structurally new about the genres which are emerging from computer culture. These include CD-ROM, World Wide Web sites, and "intranet" office-information systems. The latter are mini-Webs which use "hyperlinks", highlighted elements in a text which give access to other documents. Though the least familiar at present, intranets will be the key to the success of their companions, since they will familiarise office workers with the principle of hypertext. You'll wonder how you ever managed without it.
So, welcome to "hypermedia". It's not a perfect term, but with luck the tinny overtones of hype and cyberbabble will die away. Hypertext is not the biggest thing since movable type, as some pundits have implied, but it is an innovation powerful enough to form the trunk of a new and distinctive tree of cultural forms.
Settling on a single term to cover the various relationships one might have with hypermedia is much harder. "Viewer" excludes reading and hearing. The fact that it is accepted as the term for a consumer of television, who also reads from the screen and listens to the programmes, reflects the dominance of images over the other dimensions of the medium. In hypermedia, a healthier balance is supposed to obtain. "Reader" is appropriate for many Web presentations, which are based on text, and its meaning could be stretched by appealing to the academic usage which speaks of "reading" images. But it seems a touch ironic when hypermedia visual effects are so often intended to dazzle rather than to be legible.
The basic metaphors of hypermedia are spatial, which is why Web "browser" programs have names like Navigator and Explorer. But these imply an unrealistic degree of purpose. We need an expression to describe meandering interspersed with short bursts of directed movement. I don't know what it's called, but it's what small creatures in ponds do.