private synchronized static Undefined[] getUndefinedTypes() {
		if (undefinedTypes == null) {
			undefinedTypes =
				new Undefined[] { Undefined1DataType.dataType, Undefined2DataType.dataType,
					Undefined3DataType.dataType, Undefined4DataType.dataType,
					Undefined5DataType.dataType, Undefined6DataType.dataType,
					Undefined7DataType.dataType, Undefined8DataType.dataType };
		}
		return undefinedTypes;
	}
<code>Undefined</code> identifies an undefined data type
Ghidra/Framework/SoftwareModeling/src/main/java/ghidra/program/model/data/Undefined.java