Waldeinsamkeit is a German word referring to the feeling of solitude when alone in the woods. These days it is most familiar to English speakers through its frequent inclusion on internet lists of words that are supposedly untranslatable into English, although it has been referenced in English literary circles for at least the last few centuries.
While English speakers are likely to assume that waldeinsamkeit refers to a particularly peaceful feeling one has when walking in the woods, historically (and in Germanic tradition) this word is more closely associated to religious hermits seeking an ascetic lifestyle, with a distinct overtone of loneliness. In later literary works waldeinsamkeit does start to drift towards introverted romantic poets writing about the spirituality of lounging in the woods and feeling a connection to God (or something of that nature). The phrase schöne Waldeinsamkeit ("beautiful solitude of the forest") might be used to help the reader understand that the solitude was being viewed in a positive light.
By far, the most famous English literary use of waldeinsamkeit was Ralph Waldo Emerson's poem Waldeinsamkeit, which is very much akin to earlier German poems such as the bird's song in Ludwig Tieck's 1797 work Eckbert the Blond (Wikisource).
Ralph Waldo Emerson, 1858
I do not count the hours I spend
In wandering by the sea;
The forest is my loyal friend,
Like God it useth me.
In plains that room for shadows make
Of skirting hills to lie,
Bound in by streams which give and take
Their colors from the sky;
Or on the mountain-crest sublime,
Or down the oaken glade,
O what have I to do with time?
For this the day was made.
Cities of mortals woe-begone
Fantastic care derides,
But in the serious landscape lone
Stern benefit abides.
Sheen will tarnish, honey cloy,
And merry is only a mask of sad,
But, sober on a fund of joy,
The woods at heart are glad.
There the great Planter plants
Of fruitful worlds the grain,
And with a million spells enchants
The souls that walk in pain.
Still on the seeds of all he made
The rose of beauty burns;
Through times that wear and forms that fade,
Immortal youth returns.
The black ducks mounting from the lake,
The pigeon in the pines,
The bittern's boom, a desert make
Which no false art refines.
Down in yon watery nook,
Where bearded mists divide,
The gray old gods whom Chaos knew,
The sires of Nature, hide.
Aloft, in secret veins of air,
Blows the sweet breath of song,
O, few to scale those uplands dare,
Though they to all belong!
See thou bring not to field or stone
The fancies found in books;
Leave authors' eyes, and fetch your own,
To brave the landscape's looks.
Oblivion here thy wisdom is,
Thy thrift, the sleep of cares;
For a proud idleness like this
Crowns all thy mean affairs.