Quote webster
im·mor·tal adjective \(ˌ)i-ˈmȯr-təl\
: not capable of dying : living forever
—used to say that something will last or be remembered forever
Full Definition of IMMORTAL
1
: exempt from death <the immortal gods>
2
: exempt from oblivion : imperishable <immortal fame>
3
: connected with or relating to immortality
4
: able or tending to divide indefinitely <immortal cell lines produced in culture>
http://www.merriam-webster.com/dictionary/immortal
before you go and say example 4 is what you are referring to, no. this instance is related to a certain jellyfish(forget which one) which, basically, has its cells create indefinite clones of themselves. this is the only known example, in nature, in which an animal can basically restart its life cycle, it goes back to the stage before it is sexually mature and creates the clones. i forget exactly the structure set up(marine biology class in highschool, live in florida so it was an option)