an educated guess about the future outcome of an experiment or observation; may be framed as a detailed question or statement 4 in physics, a limited and defined set of parameters including space and matter a statement of scientific reality that appears to be true under many different circumstances a system that can be affected by the outside environment, by an exchange of matter or energy a scientific theory that has been tested repeatedly but not refuted; not immutable and may be altered as new data becomes available a system isolated from the outside environment, which cannot exchange matter or energy with the system an explanation or model of related natural events that can be tested by observations or experiments an artificial representation of a real system or process, designed to help understand the structure or behavior of the actual system or process 1. hypothesis 2. law 3. principle 4. theory 5. closed physical system 6. model 7. open physical system 8. physical system