1.
camp; battle formation; position
A military encampment or the arrangement of troops on a field. Survives mainly in set phrases and historical writing, and in figures of speech drawn from them.
2.
corps; lineup; body of people (as a suffix)
As a suffix on a noun, the assembled group of people in one role: 報道陣 (the press corps), 首脳陣 (the top brass). This is by far the commoner use in modern Japanese.