A raise of partner’s suit by two levels (e.g. 1♥-Pass-3♥). In Acol, this is a limit raise usually showing four-card support and eight losers or 10-12 points.
Opener can also make a double raise (e.g. 1♣-Pass-1♥-Pass-3♥). This would show four-card support and better than a minimum opening (six losers or 15-17 points).