In music, what is the "lost chord"?

There isn't a real thing called a lost chord. It's just that this poem, once set by Sullivan, has entered into the language as a phrase for something achieved once and evermore unattainable.

The Lost Chord
Music by Sir Arthur Sullivan;
words by Adelaide Anne Proctor
Song Lyrics:

Seated one day at the organ, I was weary and ill at ease,
And my fingers wander'd idly over the noisy keys;
I knew not what I was playing, or what I was dreaming then,
But I struck one chord of music like the sound of a great Amen.

It flooded the crimson twilight like the close of an Angel's Psalm,
And it lay on my fever'd spirit with a touch of infinite calm.
It quieted pain and sorrow like love overcoming strife,
It seem'd the harmonious echo from our discordant life.

It link'd all perplexed meanings into one perfect peace
And trembled away into silence as if it were loth to cease;
I have sought, but I seek it vainly, that one lost chord divine,
Which came from the soul of the organ and enter'd into mine.

It may be that Death's bright Angel will speak in that chord again;
It may be that only in Heav'n I shall hear that grand Amen!

http://www.james-joyce-music.com/songb_19_lyrics.html

The Lost Chord is a poem by Adelaide Anne Procter that was set to music by Arthur Sullivan in 1877. Sullivan composed "The Lost Chord" at the bedside of his brother Fred during Fred's last illness. The manuscript is dated January 13 1877, and Fred Sullivan died five days later.

answers.com/lost%20chord webpage no longer exists

Jimmy Durante recorded a song called "The Guy Who Found the Lost Chord,"

"An organist, while practicing one day, hit a particular chord which elevated his consciousness and he experienced what might be called nirvana or a glimpse of heaven -- perhaps a trance of sorts. However, as with all forms of stimulants, it was short lived and he moved one hand and could not find that special chord again. He spent the rest of his life searching for it."

https://www.youtube.com/watch?v=vla0V2jrtYE