As some others have rightly defined it, hypotension is low blood pressure.
With high blood pressure
we define "high" by numbers. With low blood pressure, we define it by symptoms. There are plenty of people walking around in peak physical health who have blood pressure of 90/60 and are asymptomatic. Theoretically low but if they are not having symptoms, then this is a medical non-issue.
Here is a list of symptoms according to the Mayo Clinic's web site.
Some people with low blood pressure are in peak physical condition with strong cardiovascular systems and a reduced risk of heart attack
and stroke. But low blood pressure can also signal an underlying problem, especially when it drops suddenly or is accompanied by signs and symptoms such as:
Dizziness or lightheadedness
Lack of concentration
Cold, clammy, pale skin
Rapid, shallow breathing
In short, you know when your hypotension is getting worse by the severity of symptoms in terms of abruptness and how much the symptoms impact your day-to-day activities.