Base pattern (sloper or block)
A basic, well-fitted pattern with no design details, used as the starting point to develop new styles.
It is called a sloper in the US and a block in the UK. There is a base pattern for each type of garment (bodice, skirt, pants, sleeve) and for each size. Patternmakers copy it and change it, for example by moving darts, adding volume or changing the neckline, instead of drafting every style from scratch. Keeping an unmodified copy lets you always go back to the original fit.
Example
A basic pants block in the base size can become jeans, joggers or shorts by changing the width of the legs, the pockets and the length.