Butterhead lettuce is a category of lettuces.
These lettuces have tender leaves, with heads that are half-open: they are compact in the centre, loosely-leafed on the outsides.
The loose leaves are flopsy.
The leaves in the centre are very soft, thus the allusion to “buttery”, and the leaves have a milder flavour than loose-leaf lettuces.
There are many varieties such as Bibb, Buttercrunch, Boston, etc.
Some have softer leaves, some have crunchier leaves.
The leaves of most butterhead-type lettuces are green, but there are varieties (such as Sangria) that have red or bronze leaves.
The leaves are easily bruised.