What does 3/8” or ¾” means in thread nomenclature?

• 3/8” or ¾” means the nominal OD of the bolt. 
• For Course threads the TPI will be 16 and for fine threads the TPI will be 20. 
• Actually that’s according to the American standards. Metric standards define it in eg. M10 * 1.5 where, 10 is the nominal diameter and 1.5 is the pitch. 
• If Pitch is not mentioned, then we take it as that thread is belonging to the course group.