Conduit is a type of raceway, and tubing is a distinctly different type of raceway.
An online search revealed this answer to that question: “A tube or trough for protecting electric wiring.” To someone untrained in the electrical field, that definition seems correct. But using that definition in electrical work causes confusion. Conduit is a type of raceway, and tubing is another (distinctly different) type of raceway.
Article 100 in the 2017 National Electrical Code (NEC) defines raceway as “An enclosed channel designed expressly for holding wires, cables, or busbars, with additional functions as permitted in this Code.”
Requirements for approved wiring methods are found in Chapter Three. There, you’ll see sequential Articles for specific types of conduit. This sequence appears right after the sequence for cable types. It begins with Art. 342 (Intermediate Metal Conduit: Type MC) and continues on through Art. 356 (Liquidtight Nonmetallic Flexible Conduit: Type LFNC). Following this is a sequence for tubing; this starts with Art. 358 (Electrical Metallic Tubing: Type EMT) and ends with Art. 362 (Electrical Nonmetallic Tubing: Type ENT).
For some applications, conduit is permitted while tubing is not. For example, compare Class II, Division 1 locations to Class II, Division 2 locations [Art. 502].