IPA: /ˈmɑːrkdɑːn/
KK: /ˈmɑrkˌdaʊn/
A decrease in the price of an item or the amount by which the price is lowered.
The store offered a markdown on all winter clothing to make room for spring styles.
Markdown is formed from "mark" (from Old English *mearc*, meaning boundary or sign) and "down" (from Old English *dūn*, meaning hill or down). The term refers to a way of marking up text in a simplified format, often for web content, hence the idea of marking text down in a structured way.
Think of 'marking' something to create a 'sign' or 'boundary' in a simplified way — that's what markdown does for text.
No commonly confused words.