In Typescript, what is the ! (exclamation mark / bang) operator when dereferencing a member?

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When looking at the sourcecode for a tslint rule, I came across the following statement:

if (node.parent!.kind === ts.SyntaxKind.ObjectLiteralExpression) {
    return;
}

Notice the ! operator after node.parent. Interesting!

I first tried compiling the file locally with my currently installed version of TS (1.5.3). The resulting error pointed to the exact location of the bang:

$ tsc --noImplicitAny memberAccessRule.ts 
noPublicModifierRule.ts(57,24): error TS1005: ')' expected.

Next I upgraded to the latest TS (2.1.6), which compiled it without issue. So it seems to be feature of TS 2.x. But the transpilation ignored the bang completely, resulting in the following JS:

if (node.parent.kind === ts.SyntaxKind.ObjectLiteralExpression) {
    return;
}

My Google fu has thus far failed me.

What is TS’s exclamation mark operator, and how does it work?

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