Surroundings
Definition
Everything outside the system you are studying. Heat lost by the system is gained by the surroundings, and vice versa.
Technical definition
The surroundings are everything external to the defined thermodynamic system. Energy exchanged across the system boundary appears with opposite sign in the surroundings, and the second law is properly stated for the system plus surroundings together.
Example
When an exothermic reaction warms the water in a calorimeter, the water is part of the surroundings gaining the heat the system released.
Common confusion
The surroundings are not just the air. They include the container, the solvent, the thermometer: everything interacting with the system that you did not include in it.