Invisibility

The Invisibility is a power-up that temporarily makes the player hard to see.

Unreal
Unlike in later games, the Unreal Invisibility pickup makes a player entirely invisible. For other players it seems like projectiles materialize out of thin air.

Unreal: Return to Na Pali
The Cloaking Device is a pickup that is a combination of Invisibility and Jump Boots, and therefore is generally more useful than the Invisibility. When a player takes the Cloaking Device, he becomes partially invisible and gains an ability of jumping higher with greatly increased air control, even 120 times more. Offline, bots notice the cloaked player only if he starts shooting. The cloak effect lasts for a very long time and even longer than it takes for another two Cloaking Devices to respawn. This pickup is automatically added to the inventory of each player in Cloak Matches.

Unreal Tournament
In UT, the Invisibility pickup makes its owner hard to see by replacing the player and weapon skins with a mostly transparent texture with moving sparkles. This effect is most efficient if the player is in front of a brighter wall in a less lit area.

Unreal Tournament 2003/2004
In UT2003 and UT2004 Invisibility is an Adrenaline combo that lasts for about 27 seconds. The effect is similar to that in UT, but harder to see on lower texture detail settings.

Unreal Tournament 3
"The invisibility powerup invokes a powerful camouflage device, making the soldier nearly (but not entirely) invisible."

- UT3 manual

When picked up, the Invisibility leaves only faint blue shadows at the player's location, which are extremely hard to see. You almost have to know that there's an invisible player to see him or her.

An important change from previous games is, that a Shield Belt carried by the player is no longer made invisible as well, but still glows its usual bright orange-yellow color. This makes it pointless for a shielded player to pick up the Invisibility, unless he or she specificly wants to prevent an opponent from taking it.

Like other power-ups, the Invisibility is dropped when the player dies or gets hit by the Impact Hammer's EMP mode.