There necessarily cannot be an exact definition of spam and there certainly is much subjective interpretation into what constitutes as spam.
Sure there can. Maybe not for you, as I believe that what you
desire for the definition of spam is 'any transaction of which I disapprove'.
while others would claim that all tx that are accepted by miners are valid and we should not ostracize or filter any types of tx's .
In my mind, this may be the best definition we can muster. If one trusts the unseen hand of the marketplace, this should be necessary and sufficient. Absent artificial restrictions, IMNSHO.
While I empathize and understand the argument, I would posit the true cost of tx is 5-10USD now and the network is heavily subsidizing all the security costs so if someone wanted to burden the network with unnecessary tx's that merely sends btc back and forth I would consider it fine if they were taking on that cost directly instead of burdening everyone else with these costs.
They are not burdening everyone else with these costs. The block reward is what it is, and it will continue being emitted regardless of the number of 'spam transactions'. Indeed, such low value transactions only amortizes this cost over more transactions, thereby reducing the sunk block reward cost of making a single transaction. Additionally, this is not a part of the problem that need be solved for the long term. As halvings go on, this part of the problem solves itself.
Let me know when Coinbase, bitpay or any other large merchant starts recommending an alt. 
