Does using snippet sentence from sites like Wikipedia, BBC, Times, Science Daily, etc. for use in a dictionary that will be published online violate copyright?
On the face of it it does, but your use might be found to fall within theFair Use exception. There are 4+1 factors which must be "balanced", and the factors are not of equal and objectively quantifiable weight. The factors are Purpose and character of use, The nature of the copyrighted work, Amount and substantiality of the portion used in relation to the copyrighted work as a whole, Effect of the use upon the potential market for or value of the copyrighted work and the fifth factor Transformativeness. Your use seems to pass muster as research and teaching; the nature of original (factual sources rather than literary sources); not substantial copying (a few scattered sample sentences, not entire paragraph or chapters) and effect on market (using your dictionary would not provide a way to obtain the same material and avoid subscription charges). But you would need to provide full details to your attorney, to get a proper analysis, since Fair Use analysis is very fact-specific.