It's very easy to confuse *parameter* with *perimeter*. Not only do the two words sound the same, they both connote a boundary or limit.

When *parameter* is used to mean "a boundary or limit," it is usually in its plural form, and often as the object of a preposition such as *within* or *beyond*:

Let's face it, managing weight loss or maintenance can be trying enough within the *parameters* of a daily routine.

Geoff Kerr, *Weight Watchers*, November 1991

Though candor sometimes encourages Bruno to "stray beyond the *parameters* of good taste," his interaction with Lydia is always convincingly portrayed as a loving, tender relationship.

Ron Charles, *The Washington Post*, 8 Feb. 2011

In geometry, *perimeter* refers to the boundary of a closed plane figure. You might remember calculating perimeters in school. If each side of an equilateral triangle measures 9 feet, then the perimeter of the triangle measures 27 feet.
In more general use, *perimeter* describes an outer stretch, as in "The college president lived in a house on the *perimeter* of the campus."

*Parameter* originally had a meaning pertaining to mathematics as well. The dictionary defines it as "an arbitrary constant whose value characterizes a member of a system (as a family of curves); *also* : a quantity (as a mean or variance) that describes a statistical population" as well as "an independent variable used to express the coordinates of a variable point and functions of them."

Some twentieth-century usage commentators objected to the use of a scientific-sounding word like *parameter* in general contexts (as in the examples above), especially to mean "a limit or boundary," theorizing that *perimeter* must have been what the speaker or writer intended.

However, *perimeter* was not often used in the phrasings that *parameter* is used in (that is, as the object of *within* or *beyond*) until much later. In fact, a word such as *bounds* or *range* or *confines* often serves as a more appropriate equivalent for *parameter* than the marginal connotations of perimeter:

But a true novel is an extended piece of fiction: Length is clearly one of its *parameters*.

Anthony Burgess, *The New York Times Book Review*, 5 Feb. 1984

Shields will then subsume that request in the overall City operating budget that he will recommend to the City Council in mid-March, and the Council will make its final decisions on the *parameters* of the budget, including any tax increases or cuts, by the end of April.

Nicholas F. Benton, *The Falls Church News Press*, 6 Jan. 2016

Writing? Check your grammar now!