Eternal means that something never ends. It is forever. Any thing that comes to an end or can be terminated is not eternal.
Life according to First John 5:12 is a relationship with Jesus Christ. This verse says,
He that hath the Son hath life; and he that hath not the Son of God hath not life.
This is spiritual life as opposed to physical life.
Therefore, when the Bible says that whosoever believeth in Him should not perish, but have eternal life, it is telling you that you have a never-ending relationship with Jesus Christ.
This is eternal life, and nothing can bring this to an end.
Once saved, always saved.