If you're interested in reading about Lily's death, its ambiguity, the various ways to interpret it, and the implications of those interpretations, go ahead and check out her "Character Analysis." What we're going to talk about here is the word. Check out the very last paragraph of House of Mirth:
[Selden] knelt by the bed and bent over her, draining their last moment to its lees; and in the silence there passed between them the word which made all clear. (2.14.36)
Before we talk about what this word is, we're going to have to go back a little ways in the novel and find where this "word" business all started. Selden starts thinking about this mystical word all the way back in Chapter Twelve, when Lily comes by to see him and, though he doesn't know it, changes her mind about blackmailing Bertha. As she readies to leave, Selden is "still groping to for the word to break the spell" of Lily's "tranced" "faculties." But he can't find it yet. Now, cut to the next morning, when Selden cheerily makes his way to Lily's place in order to propose:
He only knew that he must see Lily Bart at once – he had found the word he meant to say to her, and it could not wait another moment to be said. It was strange that it had not come to his lips sooner – that he had let her pass from him the evening before without being able to speak it. […] It was not a word for twilight, but for the morning. (2.14.4)
Exciting! Unfortunately, Lily is dead. And yet, Selden still manages to communicate with her dead body, as you see in that final paragraph we started our discussion with. He still speaks the word to her, even though she's not conscious to hear it. He speaks to her through silence.
Have we seen this before? Yes, indeed, way back in Book I, Chapter Twelve, when Selden decides (for the first time) that he's in love with Lily and wants to marry her. It's at the living portraits party, where "Selden [gives] her his arm without speaking" and Lily "[takes] it in silence." They walk through the garden and, "still without speaking," have a seat by the fountain. Considering all the social intrigues, verbal swordplay, and word games that these two play, it's almost as though they communicate better in silence. And, what the end of the novel proves, in fact, is that they only need one word – communicated in silence – to really understand each other. It's no coincidence that The House of Mirth both begins and ends with conversations between Lily and Selden, nor one that the silent conversation at the end is more effective communication than the verbal one at the novel's beginning.
Interestingly, it isn't just Selden who identifies a single word that holds the key to his relationship with Lily. Look at these passages from the moments before her death:
As she lay there she said to herself that there was something she must tell Selden, some word she had found that should make life clear between them. She tried to repeat the word, which lingered vague and luminous on the far edge of thought – she was afraid of not remembering it when she woke; and if she could only remember it and say it to him, she felt that everything would be well.
Slowly the thought of the word faded, and sleep began to enfold her. She struggled faintly against it, feeling that she ought to keep awake on account of the baby; but even this feeling was gradually lost in an indistinct sense of drowsy peace, through which, of a sudden, a dark flash of loneliness and terror tore its way. (2.13.56-7)
Nifty! Our heroine, too, zeroes in on a single "word" here – it's almost as if she and Selden have some sort of mystical, psychic connection. Which is all great, but get to the point already, right? What is the word?
Hard to say. If you want to wrap this up and slap a bow on it, you could read an essay by scholar James Gargano called "House of Mirth: Social Futility and Faith." He argues that the "word" really is just one single word, and that that single word is "faith." Lily has to learn to take a leap of faith by marrying Selden, by living on the edge of the abyss of poverty and still feeling safe, the way that Nettie Struther has managed to do. She needs to have faith in her own morality – faith that she can live and not give in to the temptation of Rosedale's offer. She needs to have faith in her own usefulness, perhaps as a mother, given the recent events surrounding Nettie's baby.
This is by no means the only answer, nor is it clear that Wharton intended any one single word. "The word" could be a set of ideas or concepts, a feeling or a mood that both Selden and Lily grasp at the novel's conclusion. What we do know about this "word" is that it's a positive one; Selden remarks that it is "not a word for twilight, but for the morning." The word acts as a light in the darkness to Lily, something "vague" but also "luminous." And, as you see in the passages above, the idea of the "word" in Lily's mind transitions into the idea of Nettie Struther's baby. (Think: re-birth, rejuvenation, hope for the future.)
So maybe it has to do with maternity or hope or, as Gargano says, perhaps faith. Maybe it has something to do with Selden and Lily finally admitting their love to each other – something they were never really able to do in life. (Remember Lily saying, "Love me, love me – but don't tell me so!") In short, we can't tell you definitively what this word is – though we can now say with fair certainty that it is not "marshmallow." Word.