a period of seven consecutive days starting on Sunday
"The company's fiscal week begins every Sunday at midnight and ends the following Saturday night."
Any period of seven consecutive days.
"Regardless of which day you start counting, a week is always any period of seven consecutive days."
In plain English: A week is a group of seven days that repeats over and over again.
"I usually do my laundry once a week."
Usage: Use "week" to refer to any standard block of seven consecutive days, regardless of whether it starts on Sunday or Monday depending on your calendar system. Avoid confusing the singular noun with its plural form when counting multiple periods in a sentence like "three weeks."
The word "week" comes from Old English, where it originally meant a sequence of days. Its ultimate root traces back to a Proto-Indo-European term meaning "to bend or curve."