Buckling rubber sleeve
Buckling rubber sleeve is a commonly-used term to describe a variation on the rubber dome. A buckling rubber sleeve resembles an open-top rubber dome, which is placed under the keycap instead of inside the keyboard. While rubber domes by necessity always face the same way up (hence the term "dome"), buckling rubber sleeves can be mounted either way up.
Buckling rubber sleeves are not a part of the switch's contact mechanism; they provide tactility only.
Various switches use buckling rubber sleeves, including: