A step in a process that dwarfs all other steps in the process in magnitude and/or complexity.
Originated from Google Maps in which a request to "Get directions" from New York to Paris yields 39 steps, 38 of which are simple driving directions with step 23 being swimming three-thousand four-hundred and sixty-two miles across the Atlantic Ocean.
If you want to become president you must choose a name to run under. Also you must choose your campaign slogan. Step 23 is to get tens of millions of Americans to vote for you.