| Acronym | Meaning |
| CIRES | Cooperative Institute For Research In Environmental Sciences |
| CIRRIS | Cryogenic InfraRed Radiance Instrument For Shuttle |
| CIRRUS-1A | Cryogenic IR Radiance Instrumentation For Shuttle |
| CIRS | Composite Infrared Spectrometer |
| CIS | Chemical Information System |
| CIS | Commonwealth Of Independent States |
| CIS | Characteristic Isochromat Spectroscopy |
| CISA | Continuum Intermediate-state Approxn. (for Scattering) |
| CISC | Configuration Interaction (with) Size-consistency (correction) |
| CISD | Configuration Interaction With Single And Double Excitation |
| CISDQ | Configuration Interaction With Single |
| CISDT | CI With Single And Double And Triple Excitations |
| CISDTQ | Configuration Interaction With Single |
| CISP | Canadian Indoor Source Profile |
| CISS | Chromosome In Situ Suppression |
| CISS | Constant-initial-state Spectroscopy |
| CISTI | Canadian Institute For Scientific And Technical Information |
| CIT | California Institute Of Technology |
| CIT | Cylindrical Ion Trap |
| CITAC | Cooperation On International Traceability And Analytical Chemistry |
| CITE | Chemical Instrumentation Test And Evaluation |
| CITP | Capillary Isotachophoresis |
| CITS | Current-imaging Tunneling Spectroscopy |
| CIV | Critical Ionization Velocity (ionosphere) |
| CIVA | Charge-induced Voltage Alteration |
| CIVB | Configuration Interaction Valence Bond |
| CIWEM | Chartered Institution Of Water And Environmental Management |
| CJCP | Corrected Coupled J Cross-polarization Spectra |
| CJD | Creutzfeldt-Jakob Disease |
| CJII | Canadian Joint Implementation Initiative [with Regard To Greenhouse Gas Control] |
| CJO | Chemical Journals Online |
| CK | Creatine Kinase |
| CK1 | Casein Kinase 1 |
| CKD | Cement Kiln Dust |
| CKRC | Cement Kiln Recycling Coalition |
| CL | Cathode Luminescence |
| CLA | Conjugated Linoleic Acid |
| CLAES | Cryogenic Limb Array Etalon Spectrometer |
| CLAMP | Center For Laser Aided Materials Processing |
| CLBO | Cesium Lithium Borate |
| CLC | Clean Lakes Clearinghouse |
| CLC | Capacity Limiting Constituents |
| CLC | Chloride Channel |
| CLC | Cholesteric Liquid Crystal |
| CLC | Coordinated List Of Chemicals |
| CLCC | Ceramic Leaded Chip Carrier |
| CLCR | Creatinine Clearance |
| CLD | Certainly Lethal Dose |
| CLD | Chronic Lung Disease |
| CLE | Competitive Ligand Exchange |