- Sintering
- Sinuosity
- Sinuous
- Sinus arrest
- Sinus arrhythmia
- Sinus bradycardia
- Sinus lift
- Sinus rhythm
- Sinus tachycardia
- Sinuses
- Sinusitis
- Sinusoidal plane wave
- Sip-and-puff
- Siphons
- Siri
- Sirius Red
- Sirt2
- Sirtuin 2
- Sisal
- Sisic
- Sister chromatid exchange
- Sisyphus cooling
- Sitagliptin
- Site manager
- Site plan
- Site-directed mutagenesis
- Site-specific recombination
- Situated
- Situated learning
- Situation calculus
- Six Sigma
- Six-bar linkage
- Six-dimensional
- Six-dimensional space
- Six-stroke engine
- Size effect
- Skaergaard intrusion
- Skarn
- Skatole
- Skeg
- Skeletal fluorosis
- Skeletal muscle
- Skeletal muscle pump
- Skeleton
- Skeletonization
- Sketchpad
- Skeuomorphic
- Skew coordinates
- Skew-symmetric matrix
- Skewness
- Skid-steer loaders
- Skidder
- Skills
- Skimmers
- Skin
- Skin appendages
- Skin biopsy
- Skin cancer
- Skin color
- Skin condition
- Skin discoloration
- Skin effect
- Skin flora
- Skin fold
- Skin friction
- Skin friction drag
- Skin graft
- Skin grafting
- Skin lesions
- Skin maceration
- Skin prick test
- Skin repair
- Skin temperature
- Skin trauma
- Skins
- Skip
- Skiving
- Skull
- Skull fractures
- Skutterudite
- Skyglow
- Skyhook
- Skylake
- Skyrmions
- Skyscrapers
- Skywave
- Slab allocation
- Slab pull
- Slabs
- Slack bus
- Slack tide
- Slack variable
- Slag
- Slaking
- Slant range
- Slash-and-burn
- Slate
- Slate and stylus
- Slater determinants
- Slater-type orbitals
- Slates
- Slats
- Slave clock
- Sleds
- Sleep
- Sleep apnea
- Sleep debt
- Sleep deprivation
- Sleep efficiency
- Sleep medicine
- Sleep mode
- Sleep spindles
- Sleep tracking
- Sleeping sickness
- Sleeve
- Sleeve gastrectomy
- Sleeve valves
- Slender-body theory
- Slenderness ratio
- Slerp
- Slew
- Slew rate
- Slewing
- Slice sampling
- Sliced inverse regression
- Slicer
- Slicing software
- Slick
- Slickensides
- Slide rules
- Slide valves
- Sliders
- Sliding
- Sliding mode control
- Slime layer
- Slip
- Slip angle
- Slip casting
- Slip factor
- Slip forming
- Slip ratio
- Slip rings
- Slipform
- Sliplining
- Slipped capital femoral epiphysis
- Slipperiness
- Slipstream
- Slipways
- Slit lamp
- Slitting