The title of Bertrand Russell and Alfred North Whitehead's mammoth 1912 attempt to bring all arithmetic under a formal axiom system.

One curious aspect of Principia Mathematica is that it does not prove 1 + 1 = 2 until page 362.
The name Principia Mathematica is also used to refer to Sir Isaac Newton's landmark work Principia Mathematica Philosophiae Naturalis.