noun. a meridian that passes through the observer's zenith.
adjective. ['mɝˈɪdiːən'] being at the best stage of development.
noun. ['mɝˈɪdiːən'] an imaginary great circle on the surface of the earth passing through the north and south poles at right angles to the equator.
noun. ['mɝˈɪdiːən'] the highest level or degree attainable; the highest stage of development.