N+1 redundancy

N+1 redundancy is a form of resilience that ensures system availability in the event of component failure. Components (N) have at least one independent backup component (+1). The level of resilience is referred to as active/passive or standby as backup components do not actively participate within the system during normal operation. The level of transparency (disruption to system availability) during failover is dependent on a specific solution, though degradation to system resilience will occur during failover. It is also possible to have N+1 redundancy with active-active components, in such cases the backup component will remain active in the operation even if all other components are fully functional, however the system will be able to perform in the event that one component is faulted and recover from a single component failure. An active-active approach is considered superior in terms of performance and resiliency.
Examples of N+1 redundancy:

Connecting devices (server etc.) in dual switch SAN (Storage area network) fabrics employ a discrete path to each switch.

Only one path is active at any given time, resiliency is provided by the availability of an additional path if the active path becomes unavailable.

Data centre power generators that activate when the normal power source is unavailable.

1+1 redundancy typically offers the advantage of additional failover transparency in the event of component failure. The level of resilience is referred to as active/active or hot as backup components actively participate with the system during normal operation. Failover is generally transparent (disruption to system availability) as failover does not actually occur (just degradation to system resilience) as the backup components were already active within the system.
Examples of 1+1 redundancy:

Dual active power supplies in a server.

Mirrored hard drives within a server/PC system.