Cortex-A53 (formerly Apollo) is an ultra-high efficiency microarchitecture designed by ARM Holdings as a successor to the Cortex-A9. The Cortex-A53, which implemented the ARMv8 ISA, is typically found in entry-level smartphone and other embedded devices. Often A53 cores are combined with higher performance processors (e.g. based on Cortex-A57) in big.LITTLE configuration to achieve better energy/performance.
Note that this microarchitecture is designed as a synthesizable IP core and is sold to other semiconductor companies to be implemented in their own chips.