General:
* Sample
* Example
* Instance
* Exhibit
* Model
Biological:
* Organism
* Creature
* Subject
* Test subject
* Tissue
Medical:
* Biopsy
* Culture
* Smear
* Slide
Other:
* Item
* Piece
* Object
* Thing
To choose the best word, consider:
* What kind of specimen is it? (biological, geological, etc.)
* What is the purpose of the specimen? (study, display, analysis, etc.)
* What is the tone of your writing? (formal, informal, scientific, etc.)