What is the correct definition and usage of the acronym "ETA"? [closed]

Solution 1:

ETA appears to be defined as the estimated time of arrival.

It's fine to use either format. ETA is used to answer the question, "When is X expected to arrive?", which can be answered either as, "In 20 minutes" or "At 4:30 PM". In other words, either usage is correct.

Solution 2:

It's of, but that doesn't matter, because that allows both uses.

The estimated time of arrival is 10:37.

Fine, it's estimated that the person, item, etc. will arrive at 10:37.

The estimated time of arrival is 20minutes.

Fine, there's an implied context of the current time, so it's the same as "The estimated time of arrival is 20minutes from now."