Who is considered a native speaker of English? I am a little confused by the various answers found online.
A "native speaker of English" refers to someone who has learned and used English from early childhood. It does not necessarily mean that it is the speaker's only language, but it means it is and has been the primary means of concept formation and communication. It means having lived in a truly English-speaking culture during one's formative years, so that English has been absorbed effortlessly as by osmosis.
One can have been born and grown up in a country that lists English as one of its official languages and not be a "native" speaker. For example, Canadians from Quebec cannot automatically be considered native English speakers even though many speak English quite well; they were brought up speaking French as a first language and think in French (or Canardien, as I have heard unkind Parisians refer to it). But the rest of Canada does largely consist of native speakers of English.
Speaking "like a native" of any language means more than just knowing vocabulary and grammar. Many educated foreign speakers speak better formal English than, say, many Americans or British or Australians. But formal English is only one aspect of the language. Knowing instantly what slang means, what cultural references mean, how to reduce syntax to a bare minimum and still convey precise meaning — all these things, and more, are what constitute native speech.
It's apparent that different people have different notions of what a "native speaker" is. To a linguist, the term generally implies that a speaker has "internalised" the language through "natural acquisition", rather than through deliberate instruction/learning.
A "native speaker", as opposed to an extremely proficient second language speaker, can often make instant judgements about whether sentences "on the fringe" of the language's grammar sound grammatical. So for example, native speakers can probably instantly make judgments about whether the following sentences of English "sound normal":
Which students did you think had done their homework?
Which students did you wonder whether would turn up late?
These are the parents affected by the measures.
These are the parents baked a cake by their children.
It appears that a non-native speaker, even an extremely proficient one, will tend to make a judgement about these sentences much less readily.
There are other, essentially non-linguistic, definitions of "native speaker", e.g. "the language that I speak most and have the most cultural attachment to" or "The language that I acquired first". An issue which I'm actually currently discussing on another forum with fellow translators is that there are people claiming to be "native speakers" of English who write sentences such as "I have experience of translator since 4 years". I personally think this is an unuseful definition of "native speaker", but it shows how much confusion/variability there is.
Literally, a native speaker of English is somebody that learned English as their first language.
In reality, I would say that a native speaker of English is a speaker of English which also thinks primarily in English and which other native speakers of English would recognize as such. It's certainly a circular definition but I think that that's a key part of any definition. Trying to leave it out caused me to give an incomplete definition earlier.