Abbre'via'tions, devices for saving time and space, consisting either of shortened forms of words, or of arbitrary signs or symbols substituted for words. The most common method of abbreviating is the substitution of the initial letter for the word itself, but one or more letters are often added to prevent ambiguity. Abbreviations were in common use among the Greeks and Romans, and in the manuscripts of the Middle Ages they were so numerous as to render some works exceedingly difficult to read. Even after printing was invented, the excessive use of abbreviations continued for a time.