taru-adjective, adverb taking the 'to' particle, noun
Formal or literary term describing a state of being neatly arranged or well-organized. Often used in written descriptions.
書類が整斉と並べられている。
The documents are arranged neatly.
彼の机の上はいつも整斉としている。
His desk is always orderly.
The kanji 整 (arrange) and 斉 (equal/orderly) combine to suggest a state of being well-ordered. The exact historical derivation is uncertain; the word is a literary Sino-Japanese compound.