| Single-lens reflex camera | Single-Minute Exchange of Die | Single-nucleotide polymorphism |
| Single-party | Single-party state | Single-phase electric power |
| Single-photon avalanche diode | Single-photon emission computed tomography | Single-player video game |
| Single-sideband modulation | Single-stream recycling | Singrist |
| Singular value | Sinha (footballer) | Sinhala language |
| Siniša Mihajlović | Sink | Sinking Spring, Ohio |
| Sinningia | Sino-Soviet border conflict | Sino-Soviet conflict (1929) |
| Sino-Tibetan languages | Sinoatrial node | Sinodefence |
| Sinornithomimus | Sint Eustatius | Sint Maarten |
| Sint-Gillis-Waas | Sint-Jans-Molenbeek | Sint-Laureins |
| Sintashta | Sintering | Sinterklaas |
| Sinthaweechai Hathairattanakool | Sinton, Texas | Sinus rhythm |
| Sinus tubercle | Sinusitis | Sinusoidal Alternating current |
| Sinusotomy | Sinyal İstihbaratı Başkanlığı | SiO2 |
| Sioux | Sioux Center, Iowa | Sioux City, Iowa |
| Sioux language | Sioux Rapids, Iowa | Sipah-e-Sahaba Pakistan |
| Sipahi | Siphiwe Tshabalala | Siphon |
| Siphonaptera | Siphonophorae | Siphoviridae |
| Sipo (footballer) | SIPRI | Sipuncula |
| Siquijor | Sir Arthur Eddington | Sir Aurel Stein |
| Sir Francis Humphrys | Sir George Stokes, 1st Baronet | Sir George Trevelyan, 2nd Baronet |
| Sir H. Elliot | Sir H. M. Elliot | SIR HENRY ELLIOT |
| Sir Henry Rawlinson, 1st Baronet | Sir Henry Wellcome | Sir John Chardin |
| Sir John Retcliffe | Sir Kand | Sir Patrick Moore |
| Sir Stanley Spencer | Sir Thomas Adams Professor of Arabic | Sir Thomas Adams's Professor of Arabic |
| Sir Thomas Adams’ Professor of Arabic | Sir William Hamilton, 9th Baronet | Sir Yazjan-e Galeh Zan Abu ol Hasan Beygi |
| Sir Yazjan-e Galeh Zan Mazarai | Siracourt | Siraf |
| Siraj Al-Tall | Siraj Sikder | Sirajeddine Chihi |
| Sirajuddin Haqqani | Siren (alarm) | Siren (video game) |
| Siren, Wisconsin | Siren: Blood Curse | Sirenia |
| Siri | Sirius | Sirjan County |
| Sirkka Hämäläinen | Sirkka Turkka | Sirocco |
| Sirolimus | Sirous Dinmohammadi | Sirous Ghayeghran |
| Sirr Al-Khatim Al-Khalifa | Sirte | Sirwal |
| Sirzar, Lorestan | Sisavang Vatthana | Sisavang Vong |
| SISD | Sisimiut Airport | Sisinio González Martínez |
| Sisomicin | Sisor | Sisor barakensis |
| Sisor chennuah | Sisor pakistanicus | Sisor rabdophorus |
| Sisor rheophilus | Sisor torosus | Sisoridae |
| Sisorinae | Sisoroidea | Sisseton, South Dakota |
| Sissy Spacek | Sistan | Sistani Mahalleh |
| Sistar | Sister Bay, Wisconsin | Sisters in Crime |
| Sisters, Oregon | Sistersville, West Virginia | Sistine Chapel |
| Sisymbrium | Sisymbrium altissimum | Sisymbrium austriacum |
| Sisymbrium crassifolium | Sisymbrium damascenum | Sisymbrium maurum |
| Sisymbrium officinale | Sisymbrium orientale | Sisymbrium septulatum |
| Sisymbrium strictissimum | Sisymbrium volgense | Sit-in |
| Sit-up (exercise) | Sita Sings the Blues | Sitagliptin |
| Site Analysis | SITE Intelligence Group | Site map |
| Siti Nurhaliza | Sitka, Alaska | Sitocalciferol |
| Sitophilus | Sitophilus granarius | Sitophilus linearis |
| Sitophilus oryzae | Sitophilus vateriae | Sitophilus zeamais |
| Sitotroga cerealella | Sitra Causeway | Sitta |
| Sitta azurea | Sitta canadensis | Sitta europaea |
| Sitta krueperi | Sitta ledanti | Sitta villosa |
| Sitta whiteheadi | Sittidae | Sitting |
| Situated cognition | Situated robotics | Situational analysis |
| Situational intelligence | Sium | Sium latifolium |
| Sium sisarum | Siuntio | SIV |
| Siv Jensen | SIV visa | Sivir |
| Six Characters in Search of an Author | Six degrees of freedom | Six degrees of separation |
| Six Degrees of Separation (film) | Six Feet Under (TV series) | Six Nations Championship |
| Six nines in pi | Six Sigma | Six Thinking Hats |
| Six-Day War | Six-dimensional space | Six-party talks |
| Sixteenth note | Sixten Jernberg | Sixth and Seventh Books of Moses |
| Sixth Crusade | Sixth Dynasty | Sixty Dome Mosque |
| Sixty Six (film) | Sixty Years' War | Sixty-fourth note |
| Siyabonga Nhlapo | Siyabonga Nomvethe | Siyabonga Sangweni |
| Siyadi House | Siyah Mansur Rural District | Siyavashan Rural District |
| Siza | Siza Dlamini | Size pair |
| Siziano | Sizing | SJ-M |
| Sjaak Troost | Sjögren's syndrome | Sjef van Run |
| Sjeng Schalken | Sk8er Boi | Skagen |
| Skagerrak | Skagway, Alaska | Skanawan, Wisconsin |
| Skandar Keynes | Skander Souayah | Skandha |
| Skandia Township, Michigan | Skaneateles (village), New York | Skara Brae |
| Skardu | Skateboard | Skathi |
| Skatoony | Skaudvilė | Skechers |
| Skeletal formula | Skeletal muscle | Skeletal striated muscle |
| Skeleton | Skenderaj | Skene's gland |
| Skepticism | Sketch (drawing) | SketchUp |
| Skew lines | SKF-38,393 | Skhirat-Témara |
| Skidmore, Owings & Merrill | Skien | Skien animal |
| Skikda | Skillet | Skillet (album) |
| Skills management | Skimmed milk | Skimmer (machine) |
| Skimmer (utensil) | Skimming (reading) | Skin |
| Skin allergy test | Skin appendage | Skin cancer |
| Skin cell | Skin conductance | Skin effect |
| Skin flora | Skin prick test | Skin whitening |
| Skip Garibaldi | Skippy (film) | Skirmisher |
| Skirt | Sklodowskite | SKN St. Pölten |
| Skokie, Illinois | Skolelinux | Skopje |
| Skorupa | Skoura M'Daz | Skrunda |
| SKS | Skull | Skull fracture |
| Skunk Works | Sky | Sky blue |
| Sky Captain and the World of Tomorrow | Sky City (Changsha) | Sky Destroyer |
| Sky High (2005 film) | Sky lantern | Sky News |
| Sky prayers | Sky Sports | Sky Tower (Auckland) |
| Sky Valley, Georgia | Sky-Map.org | Skybox Imaging |
| SkyDrive | Skyfall | Skykomish, Washington |
