Library Clause
Context Clause ---- used in ----> Entity
Package
Configuration

Syntax

library library_name, another_library_name;

See LRM section 11.2

Rules and Examples

A library clause must appear before the start of a design unit: 
library IEEE;
use IEEE.STD_LOGIC_1164.all;
entity WIDGET is

-- etc.

end WIDGET;

The library clause is validonly for the design unit immediately following, and any of its secondary units.

Each simulator provides a way to map the library logical name to the library's physical position in the host environment. This is typical done through shell variables, or file system links.

The libraries work and std do not have to be declared. Each design unit is implicitly preceded by the following: 
library WORK; 
library STD;
use STD.STANDARD.all;

Synthesis Issues

The library clause is usually ignored by synthesis tools.

Whats New in '93

The library clause has not changed in VHDL-93.