Is an ESTA valid for 90 days in total or 90 days at a time?

An ESTA allows me to stay in the US for up to 90 days and it is valid for two years. I see two ways to interpret this:

1. The ESTA expires when 90 total days in the US or two years from approval have passed. If I have made four 20-day visits in the first year, I need to apply for a new ESTA to do the same thing the next year.
2. The ESTA is valid for the whole two years. There is no strict limit to how much time I can spend in the US, just that each visit be 90 days or less. (Of course the amount of travel has to be within reason; I make business trips, but I don't live there.)

Which one is correct? Or if neither is, what is the limit exactly? Most importantly, where at the ESTA pages (or some other equally official source) does it say so? I didn't manage to find any reliable information on this.

For simplicity, assume that my passport is valid for well over two years, so that plays no role.

Background

When I first got to know ESTA, my impression was that it's obviously option 1. Option 2 never crossed my mind. Of course it's safe to play assuming option 1, but I prefer not to reapply unnecessarily.

I recall receiving an expiry notice about an ESTA in the past, but getting close to 90 days on a single ESTA doesn't seem to have triggered it. This makes me think that option 2 might be the case. I'd prefer to rely on official documentation rather than guesswork.

It's apparently enough that I enter the US before my ESTA expires, but that doesn't answer my question yet.

DO I NEED TO APPLY FOR A NEW ESTA IF MY CURRENT TRAVEL AUTHORIZATION WILL EXPIRE WHILE I'M IN THE UNITED STATES?

No. ESTA travel authorization needs only to be valid upon arrival in the United States.

The most relevant questions on the ESTA FAQ page don't seem to touch upon my question. This is the closest to an answer I found, but it's not quite explicit:

WHEN DO I NEED TO REAPPLY FOR AN ESTA?

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Travel authorization approvals will typically be granted for a period of two years or until the applicant's passport expires, whichever is sooner. (…)