China 
Africa 
- Acridone
- Acriflavine
- Acrolein
- Acromioclavicular joint
- Acromion
- Acrosome
- Acrosome reaction
- Acrylamide
- Acrylate
- Acrylate polymer
- Acrylic
- Acrylic acid
- Acrylic fiber
- Acrylic resin
- Acrylic rubber
- Acrylonitrile
- Actigraphy
- Actin
- Actin-binding proteins
- Actinic keratosis
- Actinidin
- Actinin
- Actinism
- Actinium
- Actinium-225
- Actinolite
- Actinometer
- Actinomycetales
- Actinomycetes
- Actinomycosis
- Action
- Action learning
- Action potentials
- Action selection
- Action spectrum
- Activated alumina
- Activated carbon
- Activated charcoal
- Activated complex
- Activated sludge
- Activation energy
- Activation functions
- Activation products
- Activation-induced cell death
- Activators
- Active antenna
- Active appearance model
- Active chlorine
- Active contour model
- Active cooling
- Active databases
- Active design
- Active disturbance rejection control
- Active ester
- Active faults
- Active filters
- Active fire protection
- Active heave compensation
- Active ingredients
- Active laser medium
- Active layer
- Active learning
- Active load
- Active matrix
- Active matter
- Active metabolites
- Active mobility
- Active noise cancellation
- Active noise control
- Active objects
- Active perception
- Active power filter
- Active redundancy
- Active safety
- Active shape model
- Active site
- Active steering
- Active suspension
- Active transport
- Active transportation
- Active users
- Active vibration control
- Active vision
- Active-pixel sensor
- Active-set method
- Activity
- Activity coefficients
- Activity diagrams
- Activity median aerodynamic diameter
- Activity recognition
- Activity tracker
- Actor model
- Actors
- Actuarial science
- Actuators
- Acupuncture
- Acute
- Acute abdomen
- Acute bronchitis
- Acute kidney injury
- Acute leukemia
- Acute liver failure
- Acute pancreatitis
- Acute phase proteins
- Acute promyelocytic leukemia
- Acute radiation syndrome
- Acute toxicity
- Acute tubular necrosis
- Acute-phase proteins
- Acyclic
- Acyclic diene metathesis
- Acyclovir
- Acyl
- Acyl azide
- Acyl chlorides
- Acyl group
- Acyl halides
- Acyl-CoA
- Acyl-coa dehydrogenase
- Acyl-coa oxidase
- Acylation
- Acyltransferase
- Ad blocking
- Ad hoc network
- Ad hoc on-demand distance vector routing
- Ad+
- AdBlue
- Ada Lovelace
- Adaboost
- Adaline
- Adamantane
- Adaptability
- Adaptation
- Adapted
- Adapters
- Adaptive
- Adaptive Huffman coding
- Adaptive algorithm
- Adaptive chosen-ciphertext attack
- Adaptive compression
- Adaptive control
- Adaptive cruise control
- Adaptive design
- Adaptive equalizer
- Adaptive equipment
- Adaptive filter
- Adaptive histogram equalization
- Adaptive hypermedia
- Adaptive immune system
- Adaptive learning