noun
A character used to occupy a position in a data field when no meaningful data is present, such as padding in fixed-length records.
固定長ファイルでは、足りない部分に充填文字を入れる。
In fixed-length files, fill characters are inserted where data is missing.
Compound of 充填 (じゅうてん, 'filling') and 文字 (もじ, 'character'). A straightforward technical term.