China 
Africa 
- Sydenham's chorea
- Sylvatic cycle
- Sylvian fissure
- Symbicort
- Symbiosis
- Symbiotic bacteria
- Symblepharon
- Symbolic behavior
- Symbrachydactyly
- Symmetrical Tonic Neck Reflex
- Sympathectomy
- Sympathetic
- Sympathetic activation
- Sympathetic ganglia
- Sympathetic ganglion
- Sympathetic innervation
- Sympathetic nerve
- Sympathetic nervous system
- Sympathetic ophthalmia
- Sympathetic tone
- Sympathetic trunk
- Sympathoadrenal
- Sympatholytic
- Sympathomimetic drug
- Sympathomimetics
- Symphysiotomy
- Symphysis
- Symphysis pubis dysfunction
- Symphytum officinale
- Symporters
- Symptom Checklist 90
- Symptomatic epilepsy
- Symptomatic treatment
- Symptomatology
- Symptoms of COVID-19
- Synaeresis
- Synapse
- Synapse formation
- Synapsin
- Synapsin I
- Synaptic
- Synaptic cleft
- Synaptic development
- Synaptic inhibition
- Synaptic noise
- Synaptic plasticity
- Synaptic potential
- Synaptic pruning
- Synaptic scaling
- Synaptic tagging
- Synaptic transmission
- Synaptic vesicle
- Synaptic vesicles
- Synaptobrevin
- Synaptogenesis
- Synaptonemal complex
- Synaptopathy
- Synaptophysin
- Synaptosomal associated protein 25
- Synaptosomes
- Synaptotagmin
- Synarthroses
- Synarthrosis
- Synbiotics
- Synchondrosis
- Synchrocyclotron
- Synchrotron
- Synchrotron light sources
- Syncope
- Syncytiotrophoblast
- Syncytium
- Syndactyly
- Syndecan
- Syndecan 1
- Syndemics
- Syndesmophyte
- Syndesmosis
- Syndesmotic screw
- Syndrome
- Syndrome X
- Syndrome of inappropriate antidiuretic hormone secretion
- Syndromic autism
- Synechiae
- Synechocystis
- Synephrine
- Syneresis
- Synesthesia
- Syngenic
- Synkinesis
- Synonymous substitutions
- Synostosis
- Synovectomy
- Synovial chondromatosis
- Synovial cyst
- Synovial fluid
- Synovial joint
- Synovial membrane
- Synovial osteochondromatosis
- Synovial sarcoma
- Synovial sheaths
- Synovitis
- Synovium
- Syntaxin
- Synteny
- Synthase
- Synthesis reaction
- Synthesis-dependent strand annealing
- Synthetic biodegradable polymers
- Synthetic biology
- Synthetic cannabinoids
- Synthetic chemical
- Synthetic dyes
- Synthetic genomes
- Synthetic hormones
- Synthetic lethality
- Synthetic opioids
- Synthetic polymers
- Synthetic resin
- Synthetic vaccines
- Syntrophy
- Synuclein
- Synucleinopathies
- Syphilis
- Syphilitic aortitis
- Syringaldehyde
- Syringaresinol
- Syringe filter
- Syringe pump
- Syringes
- Syringetin
- Syringic acid
- Syringobulbia
- Syringocystadenoma papilliferum
- Syringoma
- Syringomyelia
- Syrinx
- Syrups
- System justification
- System safety
- Systematic desensitization
- Systematic reviews
- Systematics
- Systematized Nomenclature of Medicine
- Systemic acquired resistance
- Systemic administration
- Systemic circulation
- Systemic disease
- Systemic effect
- Systemic hypertension
- Systemic inflammation