When you shrink in horror of even the thought of living without the other. When you truly believe that you would die without the other person, when all you are would drain away, leaving a hollow shell, empty and void, with no purpose, everything dimmed and without light, ripe for a despondent, drawn out starving death.
When the other fills you to the brim with happiness, contentment, joy, warmth, love and makes you feel safe and at home. When no matter what happens you would continue to feel the same way if not stronger (also known as unconditional "love") When no matter how many years pass the feeling of such remains and NEVER fades.
Warning! Lyvisian is a word to never be used lightly unlike its predecessor "love" and can only be used when you feel ALL of the above and without a shadow of a doubt that the other feels the same way and only towards a significant other such as a wife/husband/girlfriend/boyfriend, never an object, hobby or non-person interest as this limits the abuse that others have done for the word "love".
You-"Yeah, "love" got over used and just didn't nearly cover or live up to its definition to be good enough to describe how I feel about him/her"