The Bit type is predefined in the Standard package as an enumerated data type with only two allowable values: '0' and '1'.
type bit is ('0','1');
The bit type is the basic type to represent logical values. Note that there are only two values defined for the bit type and it is not possible to use it for high impedance and other non-trivial values such as Unknown, Resistive Weak, etc. (see Std_logic).
According to the type definition, its leftmost value is '0', therefore the default value of any object of the bit type is '0'.
As the bit type is defined in the Standard package, it can be used in any VHDL specification without additional declarations.
Signals of the bit type are not resolved which means that such a signal can be assigned to an expression only once in the entire architecture.
Example 1
signal BitSig1, BitSig2 : bit;
. . .
BitSig1 <= '1';
BitSig2 <= not BitSig1;
The BitSig1 and BitSig2 signals are declared without an initial
value, therefore by default they will be assigned the '0' value. In
the next statement BitSig1 is assigned the '1' value. This value is
complemented in the following statement and is assigned to BitSig2.
Any additional assignment either to BitSig1 or BitSig2 would be illegal.
Unlike in traditional ("hand-based") digital design, logical values 0 and 1 (bit type values '0' and '1') are NOT identical to boolean values (false and true), respectively. In VHDL, the latter items form completely different type (Boolean).
Logical values for object of the bit type MUST be written in quotes to distinguish them from Integer values.