(Get your dictionary out, Syko...)Something so intangible as love rather defies true description. It isn't the same thing to one person as it will be the next.
It is entirely dependant on the situation in question, the individuals involved, and all pertinent circumstances that apply.
To more directly address your original question, however, to me anyway it is the recognition of a kindred soul within another person and the inability to live without that influence in my life on a day to day basis.
It is the failure to be able to envision my life 60 years in the future without my wife being there to share it with me (whatever that may be)
It is the willingness to endure any hardship(s) necessary to be able to be next to her even temporarily.
Were I to go on I would sound incredibly emo and seeing as I do not care to get in that particular mode today I'll stop here.
Good luck finding whatever answer(s) you are searching for.