Definition
A parameter comes from Greek para- (beside) and metron (measure): a measure set alongside, a limit you agree to work within. In the plural it usually means the boundaries of a task — 'within the parameters of this study' — the agreed scope no one should overstep. In its technical sense a parameter is a variable factor whose value you can set, like a constraint you choose rather than one imposed. Either way it defines a space: fix the parameters and you fix where the work may go.
Examples
Collocations
within the parameters of·set the parameters·define the parameters·operating parameters·key parameters
Synonyms
limit·boundary·constraint·criterion·guideline
Word family
parametric (adjective)
In TOEFL & IELTS
The classic trap is perimeter (the outer edge of a shape) versus parameter (a limit or factor) — not interchangeable. Parameter is most useful in the plural, meaning the agreed scope of a task: 'within the parameters of this essay'. Strong for Writing when you define what you will and won't cover. Set / define / stay within the parameters are the natural collocations.