Form of verb used of a future action or event, seen as prior to some moment of time which is itself still later in the future: thus, in Latin, venero 'I will have come' (will have come by e.g. the day after tomorrow) as opposed to the 'simple future' veniam 'I will come'. Thence, in English, of forms like will have come.
from The Concise Oxford Dictionary of Linguistics
