To deseasonalize is to alter a process or focus from periodical to continuous.
For instance, regarding industrial deseasonalization, the initial production of a certain coffee flavor was every other month but due to the increased demand, the production was deseasonalized. Also, regarding statistical data, the seasonal variable is removed to focus on more significant information such as a general trend.
For example, seasonal data on the annual sales indicated that revenue particularly went up on January, March, and December. However, if you want to know if the rates have generally increased, the data set will be deseasonalized to have a clearer picture of the year’s sales.