Bug 58407

I think there is good evidence to support this suggestion for captions of artwork. The default entry should read "Figure" rather than "Illustration" as the latter is widely regarded as a more generic term, often including tables. Here are references from a few prominent style manuals on definitions of these elements and captioning them:

APA Publication Manual 6th Edition
Only uses the term "Figure" throughout and uses the caption "Figure N".

Chicago Manual of Style 16th Edition
Uses the term "Illustration" throughout as a generic reference to all inclusions of artwork (but not tables). When explicitly referring to an instance (or example) the text used is "Figure" and all included images are captioned with "Figure N.N" in the manual itself. In §1.38 it states "In books containing various kinds of illustrations, the list [of illustrations] may be divided into sections headed, for example, Figures, Tables (see fig. 1.6), or Plates, Drawings, Maps." Under §2.25 the use of the "fig. N.N" form (as just quoted) for numbering illustrations is provided, and under §3.1 it states "Illustrations, also called figures, consist of artwork -which is to say, anything that is represented by means of an image [...] Illustrations, then, include paintings, photographs, line drawings, maps, charts, and examples from musical scores. This list is sometimes extended to include audio and video files ..." §§3.7-3.11 outlines numbering and captioning of illustrations using the form indicated i.e., "Figure N.N".

Oxford Guide to Style 2002
Employs similar definitions to Chicago i.e., "illustration" as a generic reference implying figures, drawings, and tables. Sometimes though it uses the expression "illustrations or figures". Under §1.2.12 it states "the list of illustrations enables the reader to locate all figures, plates, or maps in a work." Under §7.1 (general principles) it states "Do not use 'figure' where confusion between numbers and illustrations may result." Chapter 10, on illustrations, starts with "an illustration is any image prepared as artwork and printed in a text, together with a caption [...] a figure is an illustration integrated into the text, with text flowing above, below, and sometimes around it. A plate is an illustration separate from the text, often printed on glossy art paper." Under §10.2 on captioning it states "... the first illustration in Chapter 1 is Fig. 1.1, the second Fig 1.2, and so on. [...] In standard academic works 'Figure' is usually abbreviated in text references as well as in notes and parenthetical matter, in the caption itself, it is often styled in capitals and small capitals.

Prentice Hall Reference Guide 8th Edition
Uses the generic term "Visual" to refer to artwork. Under §4b it states "Visuals such as images, graphs, charts, and tables [...] Images such as photographs, diagrams, maps, and illustrations [...] Graphs and charts are used to illustrate data in visual form and explain relationships between items. [...] tables, which show relationships between items, ..." All examples, which include the forms chart, graph, map, flowchart, and table, are captioned "Figure N.N".

A Manual for Writers of Research Papers ... 7th Edition (Turabian)
Under §8.4 there is a note "we use the term 'graphics' to refer to all visual representations of evidence. Another term sometimes used for such representations is 'illustrations'. Traditionally, graphics are divided into 'tables' and 'figures'. A table is a grid with columns and rows that present data in numbers or words organized by categories. Figures are all other graphic forms, including graphs, charts, photographs, drawings, and diagrams. Figures that present quantitative data are divided into 'charts', typically consisting of bars, circles, points, or other shapes, and 'graphs', typically consisting of continuous lines." §26.3 expands a little on that list by stating "The term 'figure' refers to a variety of images, including charts, graphs, diagrams, photographs, maps, musical examples, and drawings.". Under §26.3.2 on figure numbers and captions it states "On the line below the figure, write the word 'Figure' (flush left, capitalized, in roman type), followed by the figure number (in arabic numerals), followed by a period. After a space, give the caption, usually followed by a terminal period [...] If you have many figures and many chapters, use double numeration: that is, the chapter number, followed by a period, followed by the figure number, as in 'Figure 12.4'."

In terms of flexibility and to cater for larger works including several different types of artwork I think the current Category list of "Drawing", "Illustration", "Table", and "Text" could be expanded. "Diagram", "Photograph", "Chart", "Graph", "Map", and "Score", in addition to the requested "Figure" would seem likely candidates. To re-iterate though, "Figure" should be the default.

Added bug 60360 to See Also list. Refer comment three and four in that bug for related l10n issue with term "Figure" being difficult to translate to some other languages. I feel it would be disappointing if an (automated?) l10n issue prevents this enhancement from gaining support.

Also worth looking at what is possible, with respect to a similar term change, as documented in bug 70998 (Graphic/Picture -> Image).