padding character
Computing term for a character used to fill space or align data, such as in fixed-length fields or formatting.
In fixed-length data, padding characters fill the remaining space.
The loanword パディング is more common in modern technical contexts, while 埋め草文字 is a native Japanese term with a more descriptive feel.
Compound of 埋め草 (padding, filler) and 文字 (character). The exact origin is uncertain, but it likely arose in computing contexts as a direct translation of 'padding character'.