A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be transformed, by a connecting rod and crank, into rotational force for work. The term "steam engine" is most commonly applied to reciprocating engines as just described, although some authorities have also referred to the steam turbine and devices such as Hero's aeolipile as "steam engines". The essential feature of steam engines is that they are external combustion engines, where the working fluid is separated from the combustion products. The ideal thermodynamic cycle used to analyze this process is called the Rankine cycle. In general usage, the term steam engine can refer to either complete steam plants, such as railway steam locomotives and portable engines, or may refer to the piston or turbine machinery alone, as in the beam engine and stationary steam engine.

वाष्पयान या भाप का इंजन एक प्रकार का उष्मीय इंजन है जो कार्य करने के लिये जल-वाष्प का प्रयोग करता है। भाप के इंजन अधिकांशतः वाह्य दहन इंजन होते हैं जिसमें रैंकाइन चक्र नामक उष्मा-चक्र काम में लाया जाता है। कुछ वाष्पयान सौर उर्जा, नाभिकीय उर्जा या जिओथर्मल उर्जा से भी चलते हैं।