The IEEE 1164 standard defines a package design unit that contains declarations that support a uniform representation of a logic value in a VHDL hardware description.
It was sponsored by the IEEE Design Automation Standards Committee of the Institute of Electrical and Electronics Engineers

Institute of Electrical and Electronics Engineers

The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...

 (IEEE).
The standardization effort was based on the donation of the Synopsys

Synopsys

Synopsys, Inc. is one of the largest companies in the Electronic Design Automation industry. Synopsys' first and best-known product is Design Compiler, a logic-synthesis tool. Synopsys offers a wide range of other products used in the design of an application-specific integrated circuit...

 MVL-9 type declaration.

The primary data type std_ulogic (standard unresolved logic) consists of nine character literals in the following order:

Character	Value
'U'	uninitialized
'X'	strong drive, unknown logic value
'0'	strong drive, logic zero
'1'	strong drive, logic one
'Z'	high impedance
'W'	weak drive, unknown logic value
'L'	weak drive, logic zero
'H'	weak drive, logic one
'-'	don't care

This system promoted a useful set of logic values that typical CMOS logic design could utilize in the vast majority of modeling situations. The 'Z' literal makes tri-state buffer logic easy. The 'H' and 'L' weak drives permit wired-AND and wired-OR logic. Additionally, the 'U' state is the default value for all object declarations so that during simulations uninitialized values are easily detectable and thus easily corrected if necessary.

In VHDL, the hardware designer makes the declarations visible via the following library and use statements:


library IEEE;
use IEEE.std_logic_1164.all;