What does it mean "GML is an XML grammar"?

I'm trying to get my head around the theoretical side of XML and GML. The OGC web page for GML states that:

The Geography Markup Language (GML) is an XML grammar for expressing geographical features.

But what does it mean by a "grammar"? I can't see that phrase (as a noun) used anywhere else. Is it a unique GML meaning?

I see the terms "language", "schema", "format" and "standard". Are they the same thing?

So can I say that GML is an XML schema? Or would that mean something different? If so, is there a single XSD file that defines the entire GML specification?

Answer

When someone designs a class of XML documents for representing information in a particular domain, they will sometimes call this an XML grammar, or a vocabulary, or a schema, or a document type, or even a language. The terminology isn't consistent. There's perhaps a different emphasis: calling it a schema implies that an XML Schema is the primary way in which the grammar/vocabulary is specified; but they all mean essentially the same thing.