2 Answers

As a transition word, do you mean something like, "Lest we forget what happened on September 11......"
A "transition" takes you from one thought to the next.  Right? 

adj. A superlative of little.1. Lowest in importance or rank.

2. a. Smallest in magnitude or degree.

b. Slightest or tiniest: didn't care the least bit.

adv. Superlative of little.To or in the lowest or smallest degree.

n.One that is the lowest or smallest in importance, rank, magnitude, or degree: The dinner menu is the least of my worries tonight. The least you can do is to be polite.

Idioms: at least1. According to the lowest possible assessment; not less than: waited at least an hour.

2. In any event; anyway: You might at least answer.

in the leastAt all: I don't mind in the least.

least of allParticularly not: Nobody cared, least of all the manager.