The Hayflick Limit is the maximum damage beyond which it becomes prohibitively expensive to repair an Evangelion Unit. It is first heard in Episode 20. This limit is reached as a result of the battle with Zeruel, the Fourteenth Angel in Episode 19. The incident leaves both Evangelion Unit-00 and Evangelion Unit-02 severely damaged. Funding for repairs had to be diverted from the construction of new units, as a result forcing the cancellation of two scheduled units believed to be either Evangelion Units 14-15 or the unfinished original Units 05 and 06.
The Hayflick Limit is thought to be based on the real-life Hayflick Limit discovered by Leonard Hayflick in 1965. The Hayflick Limit is a biological mechanism limiting the division of cells. The human cell limit is approximately 52 divisions, after which the cells show signs of aging and eventually die. The limit is one of the causes of aging, but is also believed to play a vital role in the prevention of cancer by limiting the number of times a cancerous cell may divide. Extensive research is taking place in an attempt to overcome this limit in hopes of extending the human lifespan.