The {{e}}

template is intended to facilitate and make uniform scientific notation numbers. To render
3.14×10−12

instead of writing:

3.14×10<sup>−12</sup>

you can write:

3.14{{subst:e|−12}}

which yields that same wikitext, and is therefore rendered the same.

Note that the parameter is text, not a number; it can include variables which can be italicized, etc. This also means that using a hyphen (-) and a minus sign (−) with negative numbers gives different results.

3.14{{subst:e|-12}} 3.14×10-12
3.14{{subst:e|&minus;12}} 3.14×10−12
3.14{{subst:e|−12}} 3.14×10−12
3.14{{subst:e|''z'' − ''y''}} 3.14×10zy

See also m:Template:e (backlinks, edit).