<p><strong>Noun</strong></p><ul><li>the back of a horse.</li><li>Geology. a low, natural ridge of sand or gravel; an esker.</li></ul><p><strong>Adverb</strong></p><ul><li>on horseback: to ride horseback.</li></ul><p><strong>Adjective</strong></p><ul><li>made or given in a casual or speculative way; approximate or offhand: a horseback estimate on the construction costs.</li></ul>