Seam allowance
The extra width of fabric added beyond the sewing line of a pattern piece, so the pieces can be joined with a seam.
The sewing line is where the stitching goes; the seam allowance is the fabric between that line and the cut edge. In industrial patternmaking it is commonly between 1 and 1.5 cm (3/8 to 5/8 in) on most seams, with a wider allowance on hems. Every piece that is joined has to use consistent allowances, or the pieces will not match when sewn.
Example
A pattern drafted on the sewing line, with a 1 cm seam allowance added all around, is cut 1 cm outside that line on every edge; sewing 1 cm from the cut edge brings the garment to the intended size.