= SI prefix
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An \b[SI prefix] supplies an exact decimal factor when attached to a <unit of measurement>. For example, milli means $10^{-3}$ and micro means $10^{-6}$; a <millimetre> and a <micrometre> are correspondingly scaled <metres>. The prefix symbol and unit symbol form one unit that can itself be raised to a power.<The International System of Units (SI), ninth edition, SI Brochure>[Chapter 3, Table 7 and rules following the table]{ref}
For an original conversion example,
$$
2500\,\mathrm{mm}=2500\times10^{-3}\,\mathrm m=2.5\,\mathrm m,
\qquad
4\,\mathrm{mm^2}=4(10^{-3}\,\mathrm m)^2=4\times10^{-6}\,\mathrm{m^2}.
$$
The <area> conversion squares the factor as well as the unit. Applying just one factor of $10^{-3}$ to that <area> would give $0.004\,\mathrm{m^2}$, a value 1000 times too large. <Scientific notation> makes the power explicit.
Prefix symbols are case-sensitive and cannot be stacked. For <mass>, prefixes attach to <gram>, because the base-unit name <kilogram> already contains a prefix. SI prefixes denote powers of ten, even in a computing context; they do not silently change into powers of two.<The International System of Units (SI), ninth edition, SI Brochure>[Chapter 3, printed pp.138–139: capitalization, compound prefixes, kilogram and decimal factors]{ref} An exact unit conversion scales any stated <measurement uncertainty> with the value; it does not make an uncertain measurement exact.
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