It can be said as the increase in overall temperature of the earth due to too much increase in CO2, CH4, N2O and other gases. These gases are increasing due to many things like deforestation, traffic pollution, and nondiscriminatory use of natural resources.
Illustrative video about global warming for kids : source- YouTube
The last year 2016 remained the hottest year on records.
Through the historical records it can be seen :