Base
Base noun - A flat bottom on which something is intended to sit.
Usage example: a tub should sit on its own base
Underside is a synonym for base in position topic. In some cases you can use "Underside" instead a noun "Base", when it comes to topics like lowest part.
Underside
Underside noun - The side or part facing downward from something.
Usage example: the underside of the cat's coat is pure white
Base is a synonym for underside in position topic. You can use "Base" instead a noun "Underside", if it concerns topics such as lowest part, lower side.
Mutual synonyms
Both words in one sentence
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During the fight in the ocean with Adiane, the extremely phallic mobile base gets its main underside cannon destroyed.
Source: Groin Attack / Anime and Manga - Underside Ride: In a flashback, it's revealed that John and his squad did this to infiltrate an enemy base during a training exercise.
Source: Literature / Halo: First Strike
Cite this Source
Underside and Base. (2016). Retrieved 2026, August 13, from https://thesaurus.plus/related/base/underside
Base & Underside. N.p., 2016. Web. 13 Aug. 2026. <https://thesaurus.plus/related/base/underside>.
Underside or Base. 2016. Accessed August 13, 2026. https://thesaurus.plus/related/base/underside.
Google Ngram Viewer shows how "base" and "underside" have occurred on timeline