It occurs when changes in Earth’s climate system result in new weather patterns that remain in place for an extended period of time. That is Wikipedia’s explanation of the term.
By extended period of time we mean anything between few decades to millions of years. The reasons for climate change can be different – internal Earth climate cycles (100-2000 years long), external – Sun activity, human activity or unexpected catastrophic events – meteorite fall, as example.
Speaking about climate change, careful and scientific approach should be followed. Before making conclusions, the one needs to check all the facts.