=== ConTeXt ===
This sets the label width to 6em:
<context>
\definedescription[notation][
headstyle=bold,style=normal,align=left,location=hanging,
width=6em]
\starttext
\notation{Label}
bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
bla bla bla bla
\notation{Very long label}
bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
bla bla bla bla bla bla bla bla bla bla bla bla bla bla bla
bla bla bla bla
\stoptext
</context>
{{Howto}}