A vertical line intersected at the middle by an equally sized horizontal line, a cross, a plus sign.
This icon is used in so many places - Add Time on a microwave oven, Increase Temperature or Time on an oven, Channel Up on TV remote controllers. I view this icon as incrementing a specific value as opposed to an up arrow that increases without understanding the exact amount of the increase. Plus - go up one. Up arrow - go up. Plus - digital. Up arrow - analog.