- Aseptic
- Aseptic bursitis
- Aseptic necrosis
- Asphyxia
- Aspirate
- Aspiration
- Aspiration pneumonia
- Aspirin
- Assistive device
- Asthma
- Astigmatism
- Ataxia
- Ataxia, cerebellar
- Ataxia-telangiectasia
- ATCC (American Type Culture Collection)
- Athetosis
- Athlete foot
- Atresia
- Atria
- Atrial arrhythmias
- Atrial fibrillation
- Atrial septal defect (ASD)
- Atrial septum
- Atrioventricular node
- Atrium
- Atrophy
- Attention deficit disorder (ADD)
- Audiogram
- Audiology
- Audiometry
- Aura
- Auricle
- Auricular
- Auscultate
- Autism
- Autoclave
- Autogenous
- Autoimmune disease
- Autonomic nervous system
- Autopsy