also called the Conqueror's Cross
On this cross IC represents the Greek characters Iota (Ι) and Sigma (Σ) - the first and last letters of Jesus (ΙΗΣΟΥΣ). (UC is sometimes seen in place of, or in addition to, IC. See below.) The letters XC represent Chi (Χ) and Sigma (Σ) - the first and last letters of Christ (ΧΡΙΣΤΟΣ).
IC XC are often accompanied with the letters NIKA, which is taken from the Greek form of In Hoc Signo Vinces, meaning 'to be victorious' or 'to conquer'. Constantine adopted this motto when he witnessed the sign (see Chi Rho cross). The ICXC Cross is therefore also known as the Conqueror's Cross.
The coin on the right is a 12th century gold tarì of Palermo or Messina seen in the Fitzwilliam Museum, Cambridge, England. Here, the IC XC NI KA surround a cross with splayed arm ends. See also Amalfi Cross.
UC is sometimes seen in place of, or in addition to, IC.
The letters U and C are Slavonic (Cyrillic) for Tse and S (the Tse looks like a U but it has a small extension down the right-hand side that is sometimes difficult to discern). The UC (Tse and S) stands for Tsar Slavy (King of Glory) and is sometimes placed as a title of Christ: 'King of the Jews'.1
A lovely example can be seen on this 18th century Russian cross (from www.russian-icon.com). At the top of this cross the U can be seen on the left of the titulus, where 'Tsar' is shown just below the angel. The C is on the right, where 'Slavy' is shown below another angel.
The main crossbeam features IC and XC at the ends, along with writing which translates as 'Bow to Your Cross and Glorify Your Holy Resurrection'
At the foot of this cross we can see the skull of Adam and some of the Arms of Christ.
Abbreviations such as these are common among Eastern Christian iconography and in the West, on Italian panels. They help focus the mind on a particular teaching and this cross teaches that Jesus Christ was victorious over death2 and through Him we can all conquer death3. (See The Meaning of the Cross)
IC XC also appears on many church emblems, such as the Orthodox Church in Italy Chiesa Ortodossa in Italia (COI), Estonian Orthodox Church Eesti Apostlik-Oigeusu Kirik (EOK), Orthodox Church of Greece (Holy Synod in Resistance) (OCG), and Slovak Greek Catholic Church (SGCC).
(Click any image to enlarge)