Full Idea
Cantor believed he had discovered that between the finite and the 'Absolute', which is 'incomprehensible to the human understanding', there is a third category, which he called 'the transfinite'.
Gist of Idea
Cantor named the third realm between the finite and the Absolute the 'transfinite'
Source
report of George Cantor (works [1880]) by Shaughan Lavine - Understanding the Infinite III.4
Book Reference
Lavine,Shaughan: 'Understanding the Infinite' [Harvard 1994], p.51