Computing term for an identifier that describes or designates something, such as a file descriptor or a designated element in a specification.
Use a file descriptor to specify which file to open.
この指定子は、要素の種類を示すために使われます。
This designator is used to indicate the type of element.
Compound of 指定 (してい, 'designation') + 子 (し, 'element' or 'child'), likely a calque from English 'designator' or 'descriptor' in computing contexts.