Google Plus search operator is no longer available and you now have to use quotation marks even for single words and search for ["ai"].
The plus operator was useful for single words and it was intuitive, considering that the minus operator is employed to exclude results that contain a certain word. If you search for [ai], Google matches terms like "artificial intelligence", "Adobe Illustrator" etc. and that's not so useful. To restrict the results to pages that include "ai", you had to replace your query with [+ai]."
According to Web Search help center,
"Google employs synonyms automatically, so that it finds pages that mention, for example, childcare for the query [ child care ] (with a space), or California history for the query [ ca history ]. But sometimes Google helps out a little too much and gives you a synonym when you don't really want it. By putting double quotes around a single word, you are telling Google to match that word precisely as you typed it."