The caution triangle ⚠︎
The caution symbol is a triangle with an exclamation mark inside: ⚠︎. Its Unicode name is WARNING SIGN (U+26A0), and it is the same character whether you call it a caution, alert or hazard sign. On real signs it is printed black on yellow, the color that means "be careful".
Plain triangles like △ or 🔺 carry no warning meaning on their own, and ⛛ is an upside-down triangle shaped like a yield sign. More shapes are on the triangle symbol page.
Caution vs warning vs danger
On safety signs these three words are not interchangeable. In the US, the ANSI Z535 sign standard ranks them by how serious the hazard is. DANGER (red) is for a hazard that will cause death or serious injury if not avoided. WARNING (orange) is for one that could cause death or serious injury. CAUTION (yellow) is for one that could cause minor or moderate injury.
All three often place the ⚠︎ triangle next to the signal word. So "⚠︎ CAUTION: wet floor" fits a slippery hallway, while an exposed high-voltage panel calls for DANGER. For prohibition and hazard signs, see the warning symbols hub.
⚠︎ vs ⚠️: text or emoji
The same character can show two ways. As text, ⚠︎ is a simple outline in the color of your font. As an emoji, ⚠️ is a filled yellow triangle. The emoji look comes from an invisible variation selector (U+FE0F) placed right after the symbol. On this site ⚠︎ copies in text style by default; open the symbol's details to choose the emoji style.
To type it, search "warning" in your phone's emoji keyboard, press Windows + . on Windows, or Control + Command + Space on a Mac. In Microsoft Word, type 26A0 and press Alt + X.