| Acronym | Meaning |
| CLEA | Collection Of Laws For Electronic Access |
| CLEC | Chiral Ligand-exchange Chromatography |
| CLEFT | Cleavage Of Lateral Epitaxial Films For Transfer |
| CLEIA | Chemiluminescent Enzyme Immunoassay |
| CLEVER | Clinical Laboratory For Evaluation And Validation Of Epidemiologic Research |
| CLF | Current Literature File |
| CLIA | Chemiluminescence Immunoassay |
| CLIA-88 | Clinical Laboratory Improvement Amendments Of 1988 |
| CLIC | Compact LInear Collider |
| CLIDE | Chemical Literature Data Extraction |
| CLIM | Climate Change Database (accessible Through OLS) |
| CLIMAP | Climate Long-range Investigation Mapping And Prediction |
| CLIP | Corticotropin-Like Intermediate-lobe Peptide |
| CLIPS | Chemical List Index And Processing System |
| CLL | Chronic Lymphocytic Leukemia |
| CLM | Contained Liquid Membrane |
| CLND | Chemiluminescent Nitrogen Detection |
| CLOD | Concn. Limit Of Detection |
| CLOPPA | Contribution From Localized Molecular Orbitals With Polarization Propagator Approach |
| CLOS | Common Lisp Object System (language) |
| CLOT | Carbon Layer Open Tubular (column) |
| CLP | Contract Laboratory Program |
| CLP | Cecal Ligation And Puncture |
| CLP | Contract Laboratory Protocols (EPA) |
| CLRC | Central Laboratory Of The Research Councils |
| CLRTAP | Convention On Long-Range Transboundary Air Pollution |
| CLS | Cathodoluminescence Spectroscopy |
| CLS | Characteristic Loss Spectroscopy |
| CLS | Classical Least-squares |
| CLS | Core Level Spectroscopy |
| CLSA | Closed Loop Stripping Analysis |
| CLSM | Confocal Laser Scanning Microscopy |
| CLSM | Controlled Low Strength Material |
| CLSV | Current Linear Sweep Voltammetry |
| CLTE | Coefficient Of Linear Thermal Expansion |
| CLTVD | Controlled Low-temperature Vacuum Dehydration |
| CLU-IN | Clean-Up Information Bulletin Board System (formerly OSWER-BBS) |
| CM | Configuration Mixing |
| CMA | Chemical Manufacturers Association |
| CMA | Cylindrical Mirror Analyzer |
| CMAQ | Community Multiscale Air Quality |
| CMAS | CaO-MgO-aluminum Oxide-silicon Dioxide |
| CMAS | Completely Mixed Activated Sludge |
| CMASN | Calcium Oxide-magnesia-alumina-silica-sodium Oxide |
| CMB | Chemical Mass Balance |
| CMB | Completely Mixed Batch |
| CMB | Core Mantle Boundary |
| CMB | Cosmic Microwave Background |
| CMBH | Capped Mesa Buried Heterostructure |
| CMBR | Cosmic Microwave Background Radiation |