Simple Definition:

Apathy is a lack of interest, enthusiasm, or concern about something. It is when a person feels indifferent or uncaring towards a particular situation or topic. Apathy can be seen when someone shows no emotion or shows little motivation to take action or make a change. For example, if a person is in apathy about climate change, they might not show any concern about reducing their carbon footprint [how much carbon they are emitting] or supporting environmental initiatives.

Very Simple Definition:

Apathy means not caring about something or feeling bored about it. For example, if a person is feeling apathy about going to a party, they might not want to go and would rather stay at home.