What is "tri-d"

TRI-D is a 3D printed metal rocket engine. University of California, San Diego (UCSD) students built the metal rocket engine using a technique previously confined to NASA, using a GPI Prototype and Manufacturing Services printer via the Direct metal laser sintering (DMLS) method. UCSD students were the first group in the world to 3D print a rocket engine of its size, other than NASA as of February 2014. The Tri-D engine cost US$6,800.