  The green valley represents the population of galaxies that are transitioning
from the actively star-forming blue cloud to the passively evolving red
sequence. Studying the properties of the green valley galaxies is crucial for
our understanding of the exact mechanisms and processes that drive this
transition. The green valley does not have a universally accepted definition.
The boundaries of the green valley are often determined by empirical lines that
are subjective and vary across studies. We present an unambiguous definition of
the green valley in the colour-stellar mass plane using the entropic
thresholding. We first divide the galaxy population into the blue cloud and the
red sequence based on a colour threshold that minimizes the intra-class
variance and maximizes the inter-class variance. Our method splits the region
between the mean colours of the blue cloud and the red sequence into three
parts by maximizing the total entropy of that region. We repeat our analysis in
a number of independent stellar mass bins to define the boundaries of the green
valley in the colour-mass diagram. Our method provides a robust and natural
definition of the green valley.
