Evenly-spaced notes played with alternating sticking. Though usually played fast, even half notes with alternating sticking would be considered a single stroke roll.
2.
Single Stroke Four
Four notes played with alternating sticking, usually as a triplet followed by a strong beat (as in the picture) or as three grace notes before a downbeat (like a ruff).
3.
Single Stroke Seven
Seven notes played with alternating sticking, usually as sextuplet followed by a strong beat.