It is 7 millimetres. Expressing a measurement as a fraction does not imply that the measurement unit should be changed. If you want 7 mm expressed as a fraction of some other units: an inch or a kilometre, for example, it is up to you to specify which other unit you want.
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One-tenth
1/1000
Do you mean what is the abbreviation for millimeter? Because that is "mm". When you write units in fractions, you will probably want to use the abbreviation.
fraction: 2/7
7 6/7 as a fraction