Final answer:
The variable deliberately altered by a scientist in an experiment is the independent variable.
Step-by-step explanation:
The variable that a scientist changes on purpose in an experiment is the independent variable. This is the factor that is manipulated to test its effect on the dependent variable, which is the factor that is observed and measured for changes in response to alterations in the independent variable.
To ensure accuracy in the results, other variables are controlled or held constant, these are known as control variables. It's important to only change one independent variable at a time to isolate its effects on the dependent variable, which helps in establishing a cause-effect relationship.