Isn't recession is when there's an actual decrease and not just a less rapid growth? I heard that for 2020, the global economic forecast is recession. The growth will be less than 2.5%. But isn't recession is when there's a decrease in GDP (for two consecutive quarters, as I recall), not just a lower growth? Why is the term applied so unusually (unusually, right?) in this case?
NOTE: the question has been modified to refer to a global recession (or I missed the global qualifier). This answer is for a national recession. The answer by @Fizz discusses the global case. I will leave my answer unchanged.

There is no international standard for a recession definition. Definitions are country-specific. For example, in the United States, the National Bureau of Economic Research (NBER) has a committee that makes recession date determination decisions, based on a number of criteria.

The NBER has a FAQ, which probably covers everything you want to know: link.

(Other countries have similar committees.)

Otherwise, people use two consecutive quarters of declining real GDP as the definition of recession. The advantage is that this is uniform across countries. Some people call this a “technical recession” in countries that have a recession-dating body (like the NBER).

Note that we need to look at quarterly GDP, annual GDP could still rise (as in the question), yet there is a recession.

For countries that grow very rapidly in real terms, some people will call a reduction in the growth rate a “a growth recession”. The developed countries no longer grow that fast (in recent decades, only some Asian economies like China grew that fast on a sustained basis), and so it does not come up very often. As such, the term is not very well defined, since it is not used much.