Back-to-back quarters of negative GDP growth define an official "recession". However, note that in these cases, the recession is deemed to have begun with the first negative quarter. It's perfectly possible that the US, for example, is already in a recession (we don't yet know Q3 GDP let alone Q4 and Q1) and it will be many months before this uncertainty is resolved.
Q4 barely started. When results of Q3 will be known you can say for sure. But word now you are in recession or not, should not change much. It is just a term that is used.
Regardless of whether Q3 is positive or negative, we will not know whether or not the economy was in recession at the time of these posts. We will certainly need to know Q4 and may also need to know Q1 before we can be sure.
You are right to notice that "recession" here is simply a function of GDP. An economy in recession is not necessarily shrinking and an economy which is shrinking is not necessarily in recession.
Indeed, GDP is roughly a measure of current consumable output and not one of overall economic size, health, or strength. When long-term investments are neglected in favour of immediate luxuries, GDP will increase. When resources are diverted towards long-term capital investments and away from short-term gratification, GDP will decrease. Independently of GDP, economic health necessitates that investment and consumption are balanced in accordance with the preferences of the people.
