it's surprisingly difficult to look up a symbol you don't remember from a textbook, since it's very hard to type without the name.
LaTeX just calls it \otimes it's a multiplication symbol inside a circle
revenant dyke 
@Canageek wiktionary has more options if that's not right: https://en.wiktionary.org/wiki/%E2%8A%97
@Canageek for future reference, i typically use https://r12a.github.io/uniview/ to get the unicode character corresponding to a given symbol (type a guess for a word which might be in the name, like "times", into the search box and click "text")
if you know the LaTeX there are probably sites which have those mappings as well, although i'm not so familiar with them
once you have the character in unicode, pasting it into wiktionary or wikipedia can be a great resource
@Lady that's the link I was missing, I was able to get the LaTeX character name, not not a Unicode representation using my tool (which lets me draw a symbol and get the LaTeX name)
usually this gives me enough to Google, but this symbols name was too generic
@Canageek there is also https://shapecatcher.com/ if you prefer drawing, but there are so many unicode lookalikes that it can be difficult (for example i got "circled crossing lanes" and "circle with superimposed x", which are not the correct characters in this case)
if all else fails you can try just browsing the mathematical symbols and operators block but i find a text search with a good guess works ~85% of the time
@Canageek
U+2297 ⊗ CIRCLED TIMES; pasting that symbol into wikipedia gets you https://en.wikipedia.org/wiki/Tensor_product