Like bats in caves, there are words which nowadays live almost entirely in the semi-darkness of footnotes. Ibid (short for ibidem) is one, denoting a reference to a work already quoted, a function also performed by op cit (work cited) and its cousin loc cit (much the same).
You are on page 3 of your endnotes before you remember that ibid is supposed to end with a period, since it is an abbreviation for ibidem ("in the same place"). What genius decided that it was worth saving a character by this practice no longer matters. What matters is that it is now three-thirty in the morning and you have to retype three pages of notes.