The phrase "monumental step" means a very big and important action or decision that has a significant impact or creates a major change. Full definition
As the very first shelter in Chicago to not only rescue, but successfully integrate FIV + cats into our general population, Felines & Canines (under the leadership of Executive Director, Abby Smith) has broken barriers and made monumental steps towards changing the way the animal welfare community views the stigma of FIV. (felinescanines.org)
Fellow parents, you already know there are so many amazing aspects of parenthood like seeing a tiny, swaddled bundle crack a smile or cheering your tyke on for those first monumental steps. (chicagoparent.com)
For many, returning to campus was a monumental step. (fortune.com)