What do we mean by flexible in flexible AC transmission system?

FACTS is defined by the IEEE as "a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters to enhance controllability and increase power transfer capability."
A flexible AC transmission system consists of static AC devices which helps in controlling the AC voltage and increasing the transmitting power.
In normal transmission lines with no FACT devices installed, the control on transmitted power is negligible and also due to losses and no control, power tranfering capability are less.
FACTS devices offer great flexibility in terms of controlling and increasing power efficiency of the transmission lines.
Since FACT devices include shunt or series or compound compensation techniques, their is a wide range of power controllability. Whatever the condition may be the power transmission can be achieved efficiently and the power quality thus is increased to great extent.
Therefore as it possible to obtain a wide range over power control it is termed as flexible.