*Definition:**  

Isoniazid is a synthetic antibiotic and antimicrobial agent, primarily used as a first-line treatment for tuberculosis (TB). It is a hydrazide of isonicotinic acid, known for its potent activity against Mycobacterium tuberculosis.  

**Functions:**  

1. **Antibacterial Action:** Inhibits the synthesis of mycolic acids, essential components of the bacterial cell wall, effectively killing or suppressing TB bacteria.  

2. **Prophylactic Use:** Prevents the progression of latent TB infections to active disease.  

3. **Synergistic Effects:** Often combined with other anti-TB drugs to enhance efficacy and reduce resistance.  

**Applications:**  

1. **Tuberculosis Treatment:** Used as a core component in TB therapy, both for active and latent infections.  

2. **Veterinary Medicine:** Applied in the treatment of TB in animals.  

3. **Research:** Studied for its potential in combating other bacterial infections and drug-resistant strains.