| Respiration | (Environmental Engineering) Energy production in which oxygen is the terminal electron acceptor, i.e. oxidation to produce energy where oxygen is the oxidizing agent. See fermentation. |
| Reversible reaction | (Environmental Engineering) A reaction in which the reactant(s) proceed to product(s), but the products react at an appreciable rate to reform reactant(s). |
| Runoff | (Environmental Engineering) The water that flows overland to lakes or streams during and shortly after a precipitation event. |
| Re-engineering - | (Software Engineering) a series of activities that transform legacy systems (with poor maintainability) into software that exhibits high quality |
| Refactoring - | (Software Engineering) changing software in a way that improves its internal structure but does not change it external behavior; often conducted iteratively as design evolves into code. |
| Regression testing - | (Software Engineering) tests that are conducted repeated to ensure that a change has not introduced side effects |
| Reliability - | (Software Engineering) a measure of the degree to which software operates reliably over some period of time |
| Requirements analysis - | (Software Engineering) a modeling activity whose objective is to understand what the customer really wants |
| Requirements engineering - | (Software Engineering) the activities required to elicit, elaborate, negotiate, specify, and validate system or software requirements |
| Resources - | (Software Engineering) anything that is required to get the project done, people, hardware, materials, information, etc. |