The word 'essential' refers to something absolutely necessary and extremely important.
Option A, crucial refers to something of great importance.
Option B, major refers to something important, serious or significant.
Option C, critical refers to something grave, having the potential to become disastrous.
Option D, unimportant refers to something lacking in importance or significance.
Thus, according to the meaning, the word 'unimportant' is the antonym of the word 'essential'.