...and they are *all* 100% correct.
A recession is two consecutive negative growth quarters in terms of GDP.
Let me just quote wikipedia for you here.
A recession is a contraction phase of the business cycle, or "a period of reduced economic activity....Some business & investment glossaries add to the general definition a rule of thumb that recessions are often indicated by two consecutive quarters of negative growth (or contraction) of gross domestic product (GDP)
You know what a rule of thumb is right? It's not a law handed down by God, but a rough guide. This is economics, not science. Even if you were to abide strictly by the 2 quarters definition, I bet you couldn't give me a reason why it's 2 quarters and not 3 or 1.