To depress a physical button or tap a button on a touchscreen and hold it down for a second. The normal "short press" (press and release right away) performs one operation, while the long press activates another. On touchscreen monitors, smartphones, tablets and smartwatches, a long and short press are technically a long and short "tap."
Apple's Force Touch/3D Touch
Features such as Force Touch and 3D Touch are physical components that sense actual pressure. However, a long press/tap is time based and can thus be implemented in most any application. See
Force Touch and
drag and drop.