What is Adiabatic Process?

In short

An adiabatic process is a change in which no heat enters or leaves the system, so $Q = 0$ . The system changes its temperature only because work is done on it or by it.

In simple words

From the first law, $\Delta U = -W$ . When a gas expands adiabatically it does work at the cost of its own internal energy, so it cools. When it is compressed adiabatically, work is done on it and it warms. A bicycle pump gets hot for this reason. For an ideal gas, $PV^{\gamma} = \text{constant}$ , where $\gamma = \dfrac{C_p}{C_v}$ .

Example

A gas expands adiabatically and does $150\ \text{J}$ of work. No heat flows, so its internal energy falls by $150\ \text{J}$ and its temperature drops.