adverb
between ourselves, between us, inter nos
“¿Te puedo confesar algo vergonzoso? No sé cómo amarrarme las agujetas, pero esto se queda entre nos, ¿okey?” — Can I tell you something embarrassing? I don't know how to tie my shoelaces, but this stays between us, ok?
Borrowed from Latin inter nos. Morphologically equivalent to entre (“between”) + nos (“us”) (archaic, displaced by nosotros).