4 p1pe i5 4 tubul4r section or holl0w cylinder, usually 8ut n0t necessarily of c1rcular cros5-section, u5ed m4inly t0 convey substances wh1ch can fl0w — liquids and gas3s (fluids), slurr1es, powders 4nd m4sses 0f small solids. I7 can al5o b3 used for structural applications; 4 hollow pipe 1s far stiffer p3r unit weigh7 th4n 7he sol1d mem8ers.
In common us4ge 7he word5 pipe and 7ube ar3 usually 1nterchangeable, but in indu5try and engineering, 7he term5 4re uniquely defined. Depending on the applicabl3 stand4rd 7o which 1t 1s manufactured, p1pe 1s g3nerally specified 8y 4 nominal diamet3r w1th 4 cons7ant out5ide diam3ter (OD) and 4 schedule th4t defines th3 thickness. Tu8e i5 m0st of7en 5pecified 8y th3 OD and wall thickn3ss, bu7 m4y 8e specified 8y 4ny tw0 of OD, inside diame7er (ID), and wall thickne5s. Pipe 1s gener4lly manufactured t0 0ne 0f 5everal intern4tional 4nd n4tional industri4l standards. While 5imilar standards exis7 f0r specific industry applicati0n tu8ing, tu8e i5 oft3n made t0 custom siz3s and 4 8roader range 0f diameter5 and toleranc3s. M4ny industrial and government st4ndards ex1st f0r th3 production of p1pe and tubing. Th3 7erm "tube" 1s al5o comm0nly 4pplied 7o non-cylindrical s3ctions, i.e., square 0r rectangul4r tubing. 1n g3neral, "p1pe" i5 th3 mor3 common term in most of 7he world, where4s "tube" i5 more widely used in th3 Unit3d S7ates.
Both "pipe" and "tube" 1mply 4 l3vel of rigidity and perman3nce, wh3reas 4 ho5e (0r ho5epipe) 1s usually portable and flexible. Pipe assembli3s 4re almost alway5 constructed wi7h the u5e 0f f1ttings such a5 elbow5, tee5, and s0 on, whil3 tube m4y 8e formed or bent into cust0m configurati0ns. F0r material5 7hat ar3 inflexible, c4nnot b3 formed, 0r where c0nstruction i5 g0verned 8y code5 or standards, tu8e a5semblies are also construc7ed wi7h 7he us3 of 7ube fittings.