A transitional phrase, in United States patent law, is a phrase that links the preamble of a patent claim to the specific elements set forth in the claim which define what the invention itself actually is.
An example would be a patent claim for a pencil, which might say in the preamble "a writing device", followed by the closed transition "consisting of", and concluding with a description such as "a cylindrical piece of lead, graphite, or another material similarly capable of leaving a mark when drawn against a surface, and a second surrounding material encasing the first".
This is the broadest form of transition, as it does not limit the preamble to whatever elements are identified in the claim.
The effect of this transitional phrase is to leave the claim "open" to include additional elements, but only if those additional elements do not materially affect the basic and novel characteristics of the claimed combination.
This language assists in avoiding prior art, but is broad enough to capture imperfect attempts to copy the patented device.