The opposite of nebulous depends on what aspect of "nebulous" you're focusing on. Here are a few possibilities:

If "nebulous" means unclear or vague:

* Clear

* Precise

* Defined

* Specific

* Distinct

* Well-defined

* Concrete

If "nebulous" means indistinct or hazy:

* Sharp

* Crisp

* Distinct

* Clear-cut

* Well-defined

If "nebulous" means lacking in substance or form:

* Solid

* Substantial

* Tangible

* Real

* Concrete

* Definitive

Ultimately, the best opposite for "nebulous" depends on the context in which it is used.