I think you saw a mathematical relation symbol and want to know what it means. The description "double slash crossed with tilde" could match a few different symbols. Below I list the most likely candidates, show how they look, give their Unicode name, LaTeX, and what they mean. If none of these match, please copy–paste the exact character (or a screenshot) and I will identify it precisely.
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∦ (U+2226) — NOT PARALLEL TO
How it looks: ∦
LaTeX: \nparallel or \not\parallel
Meaning: indicates two lines (or vectors) are not parallel. It is the parallel sign (∥) with a diagonal slash through it. -
≁ (U+2241) — NOT TILDE / NOT SIMILAR TO
How it looks: ≁
LaTeX: \not\sim
Meaning: the relation 'is similar to' (tilde) negated — i.e., 'is not similar to'. It looks like a tilde (~) with a slash through it. -
≉ (U+2249) — NOT ALMOST EQUAL TO
How it looks: ≉
LaTeX: \not\approx
Meaning: negation of "approximately equal to" (≈). Used to say two things are not approximately equal. -
≈ (U+2248) — ALMOST EQUAL TO / APPROXIMATELY EQUAL
How it looks: ≈
LaTeX: \approx
Meaning: indicates approximate equality; not a slash, but two wavy lines stacked. -
~ (U+007E) — TILDE / SIMILAR
How it looks: ~
LaTeX: \sim
Meaning: used for similarity, asymptotic equivalence, or other context-dependent relations.
How to tell which one you have:
- Is the base shape two vertical bars (parallel sign) with a diagonal slash? Then it is probably ∦ (not parallel).
- Is the base shape a single wavy horizontal tilde with a diagonal slash? Then it is probably ≁ (not similar) or ≉ (not almost equal), depending on whether the base looked like ~ or like ≈.
How to type these:
- Unicode/HTML: use ∦ for ∦, ≁ for ≁, ≉ for ≉, ≈ for ≈, and ~ for tilde.
- LaTeX: \nparallel or \not\parallel for ∦; \not\sim for ≁; \not\approx for ≉; \approx for ≈; \sim for ~.
If you can paste the exact character you saw (copy and paste it into your reply), I will tell you exactly which symbol it is and give a short example of how it's used.