DICTIONARY Extentions /add-ons needed for internet/mobile browsers.

I search through and read a lot on internet. I dont understand the meanings of many words while reading.

are there any Add-On's/extentions available in popular internet browsers so that if i select the word which i dont know, it will
show me dictionary meaning of word.

i will be happy, if it also translates the word in Hindi.

which softwares should i use..?

I want to do the same while reading content on mobile also.

please suggest.