| Surface water | (Environmental Engineering) Water which is contained in lakes, rivers, and oceans. |
| Suspended growth reactor | (Environmental Engineering) A reactor in which the microorganisms are suspended in the wastewater. Examples of suspended growth reactors are activated sludge reactors and anaerobic digesters. See attached growth reactor. |
| Synergism is the act of working together. | (Environmental Engineering) Two chemicals which are synergistic have a greater effect together than the sum of their individual effects. The effect can be either positive or negative. |
| System | (Environmental Engineering) An arbitrarily defined area or volume surrounded by a boundary and possessing specific inputs, outputs, and reactions. |
| Scheduling - | (Software Engineering) the activity that lays out a timeline for work to be conducted on a project |
| Scope - | (Software Engineering) a bounded statement of what must be accomplished |
| Security - | (Software Engineering) the ability of software to operate in a manner that is secure from internal or external attack |
| Security testing - | (Software Engineering) testing tasks that probe the vulnerability of both client-side and server-side software |
| Selective testing - | (Software Engineering) testing only a selected set of program paths and data inputs |
| Side effects - | (Software Engineering) errors that occur because of changes |