The nameif command lets you assign a name to an interface. You can use this command to assign interface names if you have more than two network interface circuit boards in your PIX Firewall. The first two interfaces have the default names inside and outside. The inside interface has a default security level of 100, the outside interface has a default security level of 0. The clear nameif command reverts nameif command statements to default interface names and security levels.
Usage Notes
1. If you change the hardware_id of the outside interface; for example, from ethernet0 to ethernet1, PIX Firewall changes every reference to the outside interface in your configuration to inside, which can cause problems with route, ip, and other command statements that affect the flow of traffic through the PIX Firewall.
2. After changing a nameif command, use the clear xlate command.
3. The inside interface cannot be renamed or given a different security level. The outside interface can be renamed, but not given a different security level.
4. An interface is always "external" with respect to another interface that has a higher security level.
http://www.cisco.com/univercd/cc/td/doc/product/iaabu/pix/pix_sw/v_62/cmdref/mr.htm#wp1026054