A stream processor's running estimate that no more events older than time T will arrive is called a blank. Events older than it that arrive anyway are blank.

Answer

A stream processor's running estimate that no more events older than time T will arrive is called a watermark. Events older than it that arrive anyway are late events.

Also accepted: late event, late data, stragglers for late events.

Why

When the watermark passes the end of a window, the window's result is emitted. A watermark that waits longer is more complete but delays results.

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