1. The common surface separating two different media in contact. A first-order interface is an interface where λ, μ, or ρ is discontinuous; a second-order interface is where dλ, dμ, or dρ is discontinuous, as where inhomogeneous media have smooth spatially varying λ, μ, or ρ. 2. The contact or connecting element between two computing machines or components by means of which information is passed between the two. Interface devices are used to reformat data (such as to convert serial data bits to parallel bits or from analog to digital form or vice-versa), to hold data until it is needed (buffer function), and sometimes to do simple operations like summing or counting. 3. To make the output of one device acceptable as input of another device.