Number of spaces that a <Tab> in the file counts for. Also see the :retab command, and the 'softtabstop' option.

Note: Setting 'tabstop' to any other value than 8 can make your file appear wrong in many places, e.g., when printing it. The value must be more than 0 and less than 10000.

There are four main ways to use tabs in Vim:

1. Always keep 'tabstop' at 8, set 'softtabstop' and 'shiftwidth' to 4 (or 3 or whatever you prefer) and use 'noexpandtab'. Then Vim will use a mix of tabs and spaces, but typing <Tab> and <BS> will behave like a tab appears every 4 (or 3) characters. This is the recommended way, the file will look the same with other tools and when listing it in a terminal.
2. Set 'softtabstop' and 'shiftwidth' to whatever you prefer and use 'expandtab'. This way you will always insert spaces. The formatting will never be messed up when 'tabstop' is changed (leave it at 8 just in case). The file will be a bit larger. You do need to check if no Tabs exist in the file. You can get rid of them by first setting 'expandtab' and using %retab!, making sure the value of 'tabstop' is set correctly.
3. Set 'tabstop' and 'shiftwidth' to whatever you prefer and use 'expandtab'. This way you will always insert spaces. The formatting will never be messed up when 'tabstop' is changed. You do need to check if no Tabs exist in the file, just like in the item just above.
4. Set 'tabstop' and 'shiftwidth' to whatever you prefer and use a modeline to set these values when editing the file again. Only works when using Vim to edit the file, other tools assume a tabstop is worth 8 spaces.
5. Always set 'tabstop' and 'shiftwidth' to the same value, and 'noexpandtab'. This should then work (for initial indents only) for any tabstop setting that people use. It might be nice to have tabs after the first non-blank inserted as spaces if you do this though. Otherwise aligned comments will be wrong when 'tabstop' is changed.

If Vim is compiled with the +vartabs feature then the value of 'tabstop' will be ignored if 'vartabstop' is set to anything other than an empty string.

(default 8)