Neti-neti is a Sanskrit expression that translates to "neither this nor that" or "not this, not this." This expression is used in Hinduism, mainly in Jnana yoga and in Advaita Vedanta (non-dualistic spiritual practice). Neti-neti is a form of analytical meditation that helps the individual understand the nature of Brahman (absolute reality) by first understanding what is not Brahman.
A "neti-neti search" is one of the key elements of Jnana yoga practice. It is an exercise in negating rationalizations and other distractions from the non-conceptual meditative awareness of reality.