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Its called large down arrow ▼ ▼
If you know how to type the character itself or you have it in a file, then you can find out is Unicode number e.g. by opening the file in Word, placing the cursor after the character and entering Alt X. This changes the character to its number. Or you can open the file in the BabelPad editor and move the cursor before the character; BabelPad displays the number at the bottom line of its window.
If you know a character from printed matter only or otherwise need to recognize a character from its graphic shape, you can use http://shapecatcher.com/ (a bit clumsy for a “filled” character like this, but easy for more normal characters).
Once you know the Unicode number, 25BC in this case, you can construct a character reference: ▼. Should you prefer decimal numbers, you can use a calculator and then use a reference like ▼. But hex numbers are generally better for readability of code, since character numbers are conventionally written in hex. (The Unicode name of this character is BLACK DOWN-POINTING TRIANGLE.)
There is no entity defined for this character in HTML 4.01 entities or even in the proposed HTML5 entities (called “named character references” there). But an entity would not be particularly useful; entity names are not very mnemonic, and (numeric) character references can be used for any character.