Etymology

backslash

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Today it means: The punctuation mark \ (a left-leaning diagonal line). (Common uses include (1) use as a delimiter in filepaths and (2) use as an escape character to mark literals.) Full definition →

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The story

From back + slash, because it is a slash “leaning back” against the direction of writing (in writing lines from left to right), in contrast to the more common (forward) slash / (“solidus, oblique, diagonal”). The actual direction of (hand-)writing these marks can be reversed, when writing from left to right, top to baseline: the backslash forwards, the (forward) slash backwards. The naming can be explained as an anthropomorphism, assuming a bar (|) standing on the baseline, facing in the direction of writing, toppling backwards or forwards, respectively. The (new) Unicode name of \ is REVERSE SOLIDUS.

Etymology from Wiktionary, CC BY-SA 4.0.