Lethologica

This is a new word for me, but bound to be an important one
to (try to!) remember as

"Lethologica refers to the state your mind is in when you have a word 'on the tip of your tongue' but can't quite articulate yourself."  

Been there, done that -
and sadly it happens with increasing frequency as I age.

The word lethologica is derived from the Greek language terms for forgetfulness (λήθη, lēthē) and word (λόγος, logos). Lēthē originates from Greek mythology; the Lethe (or River of Oblivion) was one of the rivers that flowed through the realm of Hades, from which the shades of the dead were forced to drink in order to forget their past lives on earth.

As for The Most Important words, these pretty much cover it