Hawl
In one sentenceThe passing of a full Hijri year on zakatable wealth; it is a condition for Zakat to be due, and the rate is adjusted when calculating on a Gregorian year.
Simply put
The hawl starts on the day the wealth reaches the nisab, and Zakat becomes due when a full Hijri year has passed on it.
A business whose financial year is Gregorian calculates Zakat at 2.57768% instead of 2.5%, because the Gregorian year is about 11 days longer than the Hijri year.
Example
A person's savings reached the nisab on 1 Ramadan, so the hawl completes on 1 Ramadan of the following year, when he calculates his Zakat on what he holds that day.
Don't confuse it with
NisabThe hawl is a period of time; the nisab is an amount.