> I span space,
> but only for a stretch.
A stretch of time is a term. But as this is a spatial term and one that has its bounds. It spans space.
> I am tied,
> but without constraint.
The terms are also called bounds. But they are not bound or tied and so are not constrained.
> I live in darkness,
> but with decorum.
The luminaries are not used with the terms.
A term is a minor dignity and so it has decorum.
> I know my limits.
Again another name for boundry.
> Name me.
A name is a term.