Full Idea
There are two differences of infinite numbers from finite: that they do not obey mathematical induction (both cardinals and ordinals), and that the whole contains a part consisting of the same number of terms (applying only to ordinals).
Gist of Idea
Infinite numbers are distinguished by disobeying induction, and the part equalling the whole
Source
Bertrand Russell (The Principles of Mathematics [1903], §250)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.260