I agree with your friend - a spelling error is where you regularly spell a word wrongly a spelling mistake is where you spell a word wrongly and don't notice.
Spelling error : every time your write friend you write freind and
think it's correct.
Spelling mistake : you write wierd just once and don't notice,
but you normally write weird.
Spelling mistakes are easy to make but not always easy to notice, spelling errors are unnoticeable (by you) and mean you need to learn how to spell the word correctly.
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I found this which seems to confirm my position.
Wikipedia contributors. "Error (linguistics)." Wikipedia, The Free Encyclopedia. Wikipedia, The Free Encyclopedia, 23 Oct. 2013. Web. 29 Aug. 2014.
In linguistics, it is considered important to distinguish errors from
mistakes. Distinction is always made between errors and mistakes where
the former is seen as resulting from learner's lack of proper
grammatical knowledge and the latter as being failed to utilize a
known system correctly. ...
The difference is in the degree of change applied to the object.
Enhancing leaves the main structure and functionality the same, most features intact, and generally speaking improves the object by applying small modifications.
Revamping often changes the basis, some fundamentals, some key features. Structure can undergo some changes, but the functionality generally is kept as is although we can see re-prioritizing of some elements.
Overhauling involves disassembling into constituent parts, examining each of them carefully for flaws, replacing with equivalent new parts (subsystems), in order to bring the system's performance closer to the original specification/idea.
Those words are not interchangeable. You could overhaul an engine of an aircraft to bring the engine back to its "factory new" performance. You could enhance an engine of an aircraft by replacing some heavier parts with lighter ones, for instance. You wouldn't revamp an engine. The latter most often used to describe a process for making fixes in a social/business system, like government, company management, etc. You could, however, revamp a design of an engine, which often includes technological part (manufacturing).
Best Answer
Originally, everyone spelled it centre, but because of Noah Webster's spelling reforms, people in the US started spelling it center, particularly in the last century. Although the revised spelling center has been adopted internationally to varying extents, centre is still more popular in most regions. But regardless of how you spell it, it's the same word, so it has the same meaning either way.
To get a clearer picture of which spellings are used in which countries, I turned to the Corpus of Global Web-Based English (GloWbE). After searching for both terms, I took the raw data and created a chart of percentages using the formula
center / (center + centre), which I sorted in descending order. Here's what I found:Percent of the time center is used rather than centre, by region
As you can see, center is more popular in the US, while centre is more popular in the UK. Many regions fall closer to the center of the spectrum, with India favoring centre only somewhat, and Hong Kong using both spellings in roughly equal proportions. Overall, centre is more popular in international English.
Which should you use? Well, if you're required to follow a style guide, use whichever spelling it suggests. If you have a choice, and you're writing in a region like the US or UK that strongly prefers one spelling, I suggest you use that spelling. (I don't think either spelling is ever "incorrect", but it's possible that the person grading your test might disagree with me!) Otherwise, you can spell it however you like.
Finally, I'd like to add one note about the percentages above. The tendencies at the top and bottom ends of the chart are likely to be stronger than the percentages indicate, primarily because spellings are usually respected in proper nouns regardless of region. If you talk about the Capital Centre, for example, you're likely to spell the word centre rather than center regardless of which region you're in. But since most uses of this word are not in proper nouns, the percentages should be roughly accurate, particularly toward the center of the list; it should be enough for you to decide how to spell the word.