Olive refers to a regular PC or virtual machine that is running Juniper Networks' JUNOS software. Juniper created Olive early on so they could perform testing of JUNOS during development. These days Olive is deprecated in favor of cheap, low-end M and J-series routers but is still used by people wanting to evaluate/test JUNOS or those who are studying for Juniper certifications.
In order to get an Olive up and running you require a valid copy of a JUNOS jinstall file which can be found on a "real" Juniper router and/or obtained from Juniper under a support contract.
Olive will exist as long as Juniper has no cause to eliminate it. Please don't do anything stupid like contact JTAC about Olive, pirate copies of JUNOS or otherwise be a nuisance with it. Juniper has shown a lot of respect to its customers and the community by allowing Olive to exist. Please return the respect in kind.
Olive is in no way, shape, or form supported by Juniper or JTAC.