eisegesis
noun
eis·ege·sis | \ ˌī-sə-ˈjē-səs , ˈī-sə-ˌjē- \
plural eisegeses\ ˌī-sə-ˈjē-ˌsēz , ˈī-sə-ˌjē- \
Definition of eisegesis
: the interpretation of a text (as of the Bible) by reading into it one's own ideas
— compare EXEGESIS
Not just with Jehovah's Witnesses, but all Christian denominations there have been non-Biblical beliefs introduced that were later defended by self interpretation of the Bible (e.g. see origin of Hell fire). This is a great example.
First, the Watchtower gave us the 1914 doctrine. Looking at the book of Matthew it gave Witnesses hope that Armageddon would before the generation that saw 1914 ended. Then - the generation ended and no Armageddon.
As 1914 is a core doctrine of Witnesses, the Watchtower could not allow this belief to be proved wrong. So the goal posts were moved with a new Watchtower definition of the word generation. In a sense, the Watchtower changed the word to simply mean "a time period". However, this is very nonsensical.