Flatpack is a US military standardized Printed-circuit-board surface-mount-component package. The military standard MIL-STD-1835C defines:
Flat package (FP). A rectangular or square package with leads parallel to base plane attached on two opposing sides of the package periphery.
The standard further defines different types with varying parameters which includes package body material, terminal location, package outline, lead form and terminal count.
The original Flatpack was invented by Y. Tao in 1962 (predating the Dual Inline Package by two years) while working for Texas Instruments to achieve improved heat sinking. The first devices measured 1/4 inch by 1/8 inch (3.2mm x 6.4mm) and had 10 leads.[1]