"Deltic" is a unique word, commonly used to refer to a type of diesel engine. Although there aren't many direct synonyms for "Deltic", there are a few related terms that can be used as equivalents. "Two-stroke diesel engine" is a term that can be used interchangeably with "Deltic" to describe a specific type of internal combustion engine. "Opposed piston engine" is another term used to describe a type of engine that operates similarly to a Deltic engine. Additionally, "Crosshead engine" can be used as a synonym to refer to an engine with a similar piston-crankshaft configuration as a Deltic engine. While these terms may not be exact synonyms, they still offer a way to discuss Deltic engines using related language.