Thinking of two items that are used or experienced for most humans on a daily basis that are completely fabricated abstractions of any physical traits or requirements i.e. they were created by humans as a representations of implicit social contracts for cohesion and cooperation that then became explicit and codified.

Abstraction 1 - Money

I like the majority of humans at the time of this writing spend a great deal of my time trying to obtain and use money.  However it is purely a fictional abstraction of stored social contracts.  There are no physical laws or requirements that we use it or that the abrupt disuse of it will have any impact on life.  To state in the clearest manner possible, money is an unnecessary abstraction that we have built as a group.  

Today money could exist and tomorrow it could be gone and the planets will still spin, animals will continue to exist, and people will be born and die.  The universe will continue to expand and change in wondrous manner, but money itself is simply an idea we have taken for granted because questioning it's purpose and role is to question the base roots of out complicated societies.

My personal opinion on money is it represents a stable concept of social contract that is durable (somewhat).  What does that mean?  Well in a barter or gift giving society that exists prior to money keeping track of who has taken things and what is important and valued for a cooperative species as ourselves is complicated and impossible to scale beyond our tribe sized social network, but an agreed upon value of something allows you to move between tribe relations with value and not have to build up a new set of agreements on what the value of something is.

For example I labor to produce goods say chairs, which I then sell to other people in exchange for money in my hometown.  Months later I use this stored value or money to purchase food on a vacation a continent away.  Money as an abstraction allows you to store value and use at your discretion later in an agreed upon standard manner.  It is an abbreviated language for social cooperation that exceeds the span of the actions that produced it.