What phonetic notation is Google dictionary using?

The ī symbol used in Google's rendition of hypotension is a simplified form commonly used in dictionaries. The bar on top implies it's a "long" vowel, even if you don't know IPA.

The "full" IPA form could be ɑe aɪ ɔɪ əi aː ai, or ɑ depending on whether you're British, American, Australian, Canadian, Irish, Welsh, Scottish, or whatever. You can see why Google would rather not get bogged down with all that.

Dictionaries usually include a full list of these simplified forms with examples somewhere at the front or back of the book, and often they list some of the most common ones at the bottom of each page for easy reference.

Certainly my British Collins and Chambers both use the same symbol as Google here, but I'm afraid I don't know a formal name for the entire symbol set. In fact, I rather doubt there is one, since obviously the whole reason IPA was created was to standardise things. That's when it got complicated because if you want to accurately show pronunciation, suddenly you need a whole bunch of additional symbols for something as simple as the vowel sound in sigh (sigh).

I guess here's what you're looking for http://en.wikipedia.org/wiki/Pronunciation_respelling_for_English

It seems that Google Translate uses NOAD.

I did some research and it's not really a set-in-stone system, but actually a pronunciation guide using graphemes and phonemes (using letters or letter combinations to produce a certain phonic sound). I grew up using this sort of method when learning phonetics and pronunciation, but later I learned other systems such as IPA and others. Here is a link to the PDF that shows some graphemes (-ew) and phonemes (/y+oo/ or /yoo/) that many of us, I dare say, used when learning phonics:

http://www.phonicsinternational.com/Pronunciation%20Guide%20for%20English.pdf.

I certainly hope this helps.