What is meant by 'dissipated energy'?

In a system, we can transfer energy from an energy store into one or more useful forms of energy, however this process is not 100% efficient and some energy is wasted as heat or sound - this is known as dissipated energy. It is important not to think of this energy as being destroyed, this is impossible as it violates the principle of conservation of energy. Instead, we can think of this energy as spreading out as it heats up the surroundings or is carried away by sound waves so that it is no longer useful. 
For example, when you use your mobile phone, energy is transferred from the chemical energy store of the battery into useful light and sound, however you will often notice your mobile phone becoming hot - this is energy being dissipated as heat.