To unlock Terminal Velocity, you must create 2 portals directly above and directly below you. These portals must be directly vertical to each other, or you can fall out of your constant fall. which will prevent you from getting the achievement. To make sure that they are perfectly vertical to each other, aim as high as you can, and fire a portal on the ceiling. Now, look directly down, not turning side to side at all, firing the color of portal that you didn't fire at the ceiling below your feet, as low as the game will let you. Once again, make sure that you do not turn left or right while transitioning from the ceiling portal to the portal that is below you. Once you fire the portal below you, you should be in a constant fall. Leave the game sitting there, and wait for about 2 minutes. The achievement should unlock. Before you leave your fall, I suggest you press pause and look at the achievement menu to make sure that you unlocked the achievement before you leave your constant fall as well.