I use this as a reminder.

Needed Redundancy

N+1 redundancy refers to a needed component (N) that has at least 1 backup component (+1) in the event of a failure. N+1 is referred to as active/passive or standby.

Example of N+1: a generator starting and carrying the load when normal utility power is lost. The generator is not on all the time, which would be active/active, instead it is on standby while utility power is working normally.

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1+1 redundancy is also known as active/active resilience. 

Example of 1+1: dual active power supplies in a server. Both are running at the same time therefore it makes this model a 1+1 form.

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2N redundancy means 2 times the needed component. If I needed two generators to carry the load of the building, a 2N configuration means I would have 4 generators total.