Definition: As a noun, Byzantine relates to or refers to characteristics of the ancient Byzantine Empire and the specific art and architecture from this period. Byzantine can also refer to the city of Byzantium (which is now Istanbul). Byzantine can also be synonymous with the Eastern Orthodox Church.

As an adjective, Byzantine can be used to describe a task of enormous proportions, a challenging feat, or a power struggle.

Etymology: The capital city of Turkey was previously known as Constantinople (a name given to the city when it was the capital of the Eastern Roman or Byzantine Empire). But before this, the city was known as Byzantium. 

This city was known for its political turbulence, in-fighting and mysticism. The difficult nature of the city has become synonymous with the word Byzantine. 

All architecture and characteristics from this city are also termed Byzantine. In the late 1930s, a correlation was drawn between the deception and secrecy of Soviet Union operations and the occurrences of the old Byzantine Empire. Thus, the word became popular as a description for challenging actions and tasks.

In a Sentence

Compiling his tax return was a task of Byzantine proportions.

History books pay close attention to the rise and fall of the Byzantine Empire.

Turkey is home to many examples of architecture from the Byzantine period.

Synonyms 

Complex, Elaborate, Intricate

Antonyms 

Simple, Uncomplicated