What is spiritual?

I've heard a few different definitions of "spiritual".  One use of the word is to identify material in the realm of mystery that is not religious, making it distinct from any religion.  Another use of the word denotes anything that's meaningful to a person, that has special inner meaning.  Sometimes people mix up the term "spiritual" with "belief in God," which can get messy! (What God is or could be varies so drastically depending on a person's reality. )  The word itself though, "spiritual," is evidently of-the-spirit.  What is the spirit, then?  Is it the part of us that is alive & invisible?  The energy that runs through us? The energy that runs through plants, the ocean, stars, and fire?  I often use the word "spiritual" to refer to a deep place within.  The place where there's hope or meaning made of chaos & hardship, beauty & love.  The place inside each one of us that makes us simultaneously uniquely ourselves + a mirror of one another + unified and undifferentiated.  

The observable universe is 50 billion times greater than our solar system.  The observable part, that is.  50 billion times greater than our massive solar system.  Too large to comprehend.

And yet, within me - one unobservably small person in a scope that could observe my city, let alone our solar system or the Universe! - within me,  a whole world of emotions/thoughts/ideas/feelings/dreams exists.  And so it is in you, and you, and you. . . a whole world within each of us.  Perhaps that is spiritual. 

 “You are not a drop in the ocean; you are the entire ocean in a drop.” -- Rumi