In the book The House of Mirth what is the word that they refer to in the novel when it reads: [Selden] had found the word he meant to say to her, and it could not wait another moment to be said.
Chapter 24, which is the final chapter of The House of Mirth, end with a sudden change in Lawrence Selden. "Something" has just liberated him, and it is only up to the reader to infer from the context cues what that "something" really is. It has all to do with that "word" that he has finally come upon to tell Lily.
Since it is up to the reader to infer, let us look at the cues and literary techniques that Wharton employs to help us get there. First, we can tell from the manner in which Selden is acting, that he is highly inspired by something. We learn that Selden
...felt himself thrilling with a youthful sense of adventure. He had cut loose from the familiar shores of habit, and launched himself on uncharted seas of emotion.
Therefore, Wharton is telling us that, for the first time, Lawrence Selden is letting go of his old formalities and has allowed himself to feel something humbling, and different. So detached is he from his former self that, even in the dingy environment where Lily lives now, he overlooks everything just for the sake to see Lily
...he looked up at the triple row of windows, wondering boyishly which one of them was hers...He noticed too that there was a pot of pansies on one of the window sills, .... it was inevitable that he should connect her with the one touch of beauty in the dingy scene.
All of these actions show a man who has come to the realization that he is in love. "Love", is the presumed word that Wharton wants us to infer.
The evidence of this comes in that same moment when, to Selden's utmost dismay, he is greeted by Gerty and told that Lily has passed away from what looks like an overdose of sleeping medication. Coincidentally, now is the first time in the novel where Wharton uses the word "love" freely throughout the narrative; right toward the end. This is indicative that, the ironic death of Lily coincidentally comes right about the time when Selden realizes that he loves her and, sadly, all he has left is "that word" that he finally realizes that he feels: love.
It was this moment of love, this fleeting victory over themselves, which had kept them from atrophy and extinction ...had kept alive the faith that now drew him penitent and reconciled to her side.