What is an SMR or Small Modular Reactor?

Answer

Small Modular Reactors (SMR)

According to the International Atomic Energy Agency's (IAEA) 'Terms for Describing Advanced Nuclear Power Plants' report:

'A small modular reactor is defined as an advanced reactor that produces electricity of up to 300MWe per module and is designed either as a single or multimodule nuclear power plant, which systems and components can be fabricated as modules in the factory setting then transported to site to shorten construction duration'. 

Importantly, not all 'small reactors' are Small Modular Reactors definitionally. Specifically, a 'small reactor' is any nuclear reactor with an electrical capacity not exceeding 300MWe. 

Micro Reactors

According to the International Atomic Energy Agency's (IAEA) 'Terms for Describing Advanced Nuclear Power Plants' report:

'Microreactor is a relatively new term for a reactor able to produce between 1 and 30 MW of thermal energy used directly as heat or converted to electric power.'

Sources:

IAEA