Declares a constant.const Type ConstName = Value;

Note that you cannot declare global constants. Every constant should be declared inside an entity, class, namespace, process or function.

entity Adder
{
	const int DataWidth = 32;
	
	port
	{
		in logic[DataWidth] A, B;
		out logic[DataWidth] Sum;
	}
}

entity MyEntity
{
	const int DataWidth = 32;
	
	port in logic[DataWidth] X;
}

entity MyTestbench
{
	const any TheEntity = MyEntity;

	signal logic[TheEntity.DataWidth] Sig = 0;
	
	TheEntity uut(
		X = Sig
	);
}

class DefaultPolicy
{
	const int WordSize = 32;
	const int MemorySize = 512;
}

class CompactModePolicy
{
	const int WordSize = 16;
	const int MemorySize = 128;
}

entity SomeEntity
{
	const any Policy = DefaultPolicy;

	Core.RAMs.Synchronous.SimpleRAM<Policy.MemorySize, Policy.WordSize> RAM(
		...
	);
}

Constants can be initialized with arbitrary expressions that can be computed during compilation (e.g. function calls).

int square(int a)
{
	return a * a;
}

entity SomeEntity
{
	const int SquaredSize = square(Size);
	const int Size = 4;
	
	signal logic[SquaredSize] MySig;
}

Note that the order of the declaration is not important. When THDL++ compiler starts synthesizing MySig signal it will first find the SquaredSize constant, then find the square() function and Size constant and will finally execute square(4) to get the value.