The following categories exist on the wiki, and may or may not be unused. Also see wanted categories.
- Mental health care (1 member)
- Metalektro (1 member)
- Metallurgy (1 member)
- Michael Beijer (15 members)
- Microbiology (1 member)
- Microsoft (6 members)
- Military (3 members)
- Misc. (10 members)
- Multilingual term search (9 members)
- Multitran (1 member)
- Multitran.com (1 member)
- NCC (1 member)
- NICE (1 member)
- NVWA (1 member)
- Norwegian (1 member)
- Nuffic (4 members)
- O (0 members)
- OCR (0 members)
- OECD (1 member)
- OED (2 members)
- OGL (4 members)
- OHIM (1 member)
- Obsidian (1 member)
- OmegaWiki (1 member)
- Onroerend Goed Lexicon (4 members)
- Oosthoek (1 member)
- Open Dutch WordNet (ODWN) (3 members)
- Ophthalmology (1 member)
- Ordinal numerals (1 member)
- Origin unknown (10 members)
- Original has been edited (14 members)
- Oxford University Press (OUP) (1 member)
- Oxforddictionaries.com (1 member)
- P&ID (1 member)
- PIANC (2 members)
- PR (2 members)
- PRINCE2 (1 member)
- Pages using DynamicPageList3 parser function (10 members)
- Pages using Tabber parser tag (0 members)
- Pages with broken file links (49 members)
- Palletising (7 members)
- Paper copy (1 member)
- Paper industry (1 member)
- PatTR (1 member)
- Patentese (1 member)
- Patents (18 members)
- Patitia.net (1 member)
- Pegasus (1 member)
- Peter Kutschera (1 member)
- Piracy (1 member)