Idem is a word originating in Latin and means "the same", "equal" or "Likewise". The term is commonly used in bibliographic citations, to avoid repetition of the author's name.
When the name of the work is also repeated, in addition to the author's name, the term "ibidem", which means "in the same place, in the same passage". Example:
[1] SAUSSURE, Ferdinand de, 2006, p. 50
[2] Ibid., p. 56
In a prayer, the term is used for the same purpose: avoid repetitions. In a conversation, when a person wants to convey the same opinion as someone who has just spoken, instead of repeating the same word or phrase, they can just say "idem". The word idem gave rise to the word "identical".
The ditto sign ( " ) is a typographic sign used in informal documents (usually handwritten) to indicate that the items in the top line are the same as those in the bottom line. This way it is not necessary to write the same item twice. For example:
2 cans of blue paint, brand X
3 " " " red, Y mark (in this case, the quotation marks replace the words "paint cans")