System
The specific part of the universe chosen for thermodynamic study, separated from everything else by a real or imaginary boundary.
Unit
Dimensionless
In depth
Defining the system precisely is the first step in any thermodynamic analysis, because the sign of every energy flow depends on which side of the boundary it crosses. Systems are classified by what can pass through the boundary: matter, energy, both or neither.
Examples in the real world
The contents of a reaction flask, the gas inside a cylinder, or a single living cell.