Here are some synonyms for "specific," depending on the context:

General:

* Precise

* Particular

* Definite

* Explicit

* Detailed

* Exact

* Concrete

* Clear-cut

* Pinpointed

More formal:

* Explicit

* Precise

* Distinct

* Particularized

For indicating a unique characteristic:

* Distinctive

* Unique

* Characteristic

For indicating a narrow focus:

* Focused

* Targeted

* Limited

* Narrow

The best synonym to use will depend on the specific context of your writing.