Kihei' means a mixed troop formed by retainers of the domain, the common people, and samurais excluding retainers of the domain compared with a troop (spearhead troop) organized by only retainers of the domain and samurais.
「奇兵」とは藩士・武士のみからなる部隊（撰鋒隊）に対して、藩士と藩士以外の武士・庶民からなる混成部隊の意味。