What is monolithic ?
A structure made of single block.
But what does that related to an Integrated circuit !.
In the same way IC are made of single block of silicon then we have monolithic IC ,Nowadays ICs are monolithic.
Monolithic IC :
An electronic circuit whose components, such as transistors and resistors, are etched or deposited on a single slice of semiconductor material to produce a chip.
More Reading : The first monolithic integrated circuits