Jesus H. Christ - IHS - The Christogram
In the Latin-speaking Christianity of medieval Western Europe (and so among Catholics and many Protestants today), the most common Christogram is "IHS" or "IHC", derived from the first three letters of the Greek name of Jesus, iota-eta-sigma or ΙΗΣ. Here the Greek letter eta was transliterated as the letter H in the Latin-speaking West (Greek eta and Latin-alphabet H had the same visual appearance and shared a common historical origin), while the Greek letter sigma was either transliterated as the Latin letter C (due to the visually-similar form of the lunate sigma), or as Latin S (since these letters of the two alphabets wrote the same sound). Because the Latin-alphabet letters I and J were not systematically distinguished until the 17th century, "JHS" and "JHC" are equivalent to "IHS" and "IHC".
"IHS" is sometimes interpreted as meaning Iesus Hominum Salvator ("Jesus, Savior of men", in Latin), or connected with In Hoc Signo. Some uses have even been created for the English language, where "IHS" is interpreted as an abbreviation of "I Have Suffered" or "In His Service". Such interpretations are known as backronyms. This abbreviation ("IHS") is likely the source of the joke that Jesus' full name is "Jesus H. Christ."
One of the oldest Christograms is the Chi-Rho or Labarum. It consists of the superimposed Greek letters Chi Χ; and Rho Ρ, which are the first two letters of christ in Greek. Technically, the word labarum is Latin for a standard with a little flag hanging on it, used in the army. A Christogram was added to the flag as an image of the Greek letters Chi Rho, in the late Roman period. So Christogram and labarum are not originally synonyms.
The most commonly encountered Christogram in English-speaking countries in modern times is the X (or more accurately, Greek letter Chi) in the abbreviation Xmas (for "Christmas"), which represents the first letter of the word Christ.