No, the manufacturer means "diagonal". It's a slick trick they pull because traditional television screens have curved corners and a convex surface. That way, you can't really get an accurate measurement with a ruler. Only the designer with his training in calculus knows for sure.

A agree with Santaanacanyon that the manufacturers are pulling a trick on you, but I have a slightly different take on how the trick works though. The reported diagonal measurement historically has meant viewable area. Way back in the '80s, a common monitor size was 14" viewable area. (If you are wondering, yes, I did measure my 14" with a measuring tape. Santaanacanyon is right in that the measurement was not exact, but was fairly close to the advertised area, less than 1/2" discrepency). Larger size monitors were prohibitively expensive so almost no one had them. I did get a chance to work with some of these larger CRTs, a 16" Princeton and a 19" NEC Multisync XL (all specs viewable space). Somewhere in the mid '90s I noticed that all the monitor specs started to jump an inch. Instead of 14" being the most common monitor, 15" was now most common. After measuring a few tubes, I found no actual increase in viewable area. I asked others about this and got two answers. The extra space is either the diagonal measurement of the case or the actual size of the tube, no actual added viewing space. I'm inclined to agree with the tube size answer (see below for further explanation). This way manufacturers can advertise a larger screen without it being larger and techinically still not lie to you. Some monitor boxes will have both measurements and say something like this: 17" screen, 16" viewable area. There are practical reasons for reporting the tube size and not the viewable area, though. There are many monitor manufacturers but only a few tube manufacturers. The tube manufacturer knows well it's own product dimensions and sells it to the monitor manufacturers which will add their own electronics, contols, and casing around the tube. Even though several different manufacturers may be using the same Sony tube, the end product may produce different results in viewable area. It is just easier to go with the tube size dimension even though monitor manufacturers know the viewable area dimension of the final product. Not only that, if you go to a 3rd party about monitor information, like www.monitorworld.com, they list the screen dimension in terms of tube size as well. Does this mean that there is no such thing as a 15", 19" or whatever size (viewable area) monitor? No, it does not. These size monitors do exist, but you you will have to check your own monitor to verify whether or not your monitor size is viewable area or something else. The older your monitor, the more likely it is that the advertised size is viewable area.

CRT: diagonal case corner to opposing case corner. LCD: diagonal screen corner to opposing screen corner.