Antipope: a pope set up against the one chosen by church law, as in a schism. (Webster's Dictionary)
From the time of Christ until 1958, there were 37 antipopes.
1.Saint Hippolytus
2.Novatian
3.Felix II
4.Ursinus
5.Eulalius
6.Lawrence
7.Dioscorus
8.Theodore
9.Paschal
10.Contantine
11.Philip
12.John
13.Anastasius
14.Christoher
15.Boniface VII
16.John XVI
17.Gregory
18.Benedict X
19.Honorious II
20.Clement III
21.Theodoric
22.Albert
23.Sylvester IV
24.Gregory VIII
25.Celestine II
26.Anacletus II
27.Victor IV
28.Victor VI
29.Paschal III
30.Callistus III
31.Innocent III
32.Nicholas V
33.Clement VII
34.Benedict XIII
35.Alexander V
36.John XXIII-I
37.Felix V
Since 1958 there has been a proliferation of antipopes. Some say that these men should not be called antipopes, since they were not set up against the one chosen by church law. As heretics, these men were not papabile. However, because they are claiming the Papacy we have chosen to use the term antipope and to expand the definition to meet these unique times.
All of the Antipopes prior to 1958 were of the first kind; that is elected, while a true Pope reigned. Those from 1958 to the present have been of the latter kind, although several are of both kinds (i.e. heretics elected while a true Pope reigns)
Some claim that Joseph Siri was elected in 1963 and then the claim was changed to 1958. This claim deserves mention, because of the number of people following this alleged claim.