以此 is a formal, written conjunction or adverb meaning 'therefore,' 'thus,' or 'by this means.' It combines the preposition 以 (yǐ), meaning 'by,' 'with,' or 'using,' and the demonstrative pronoun 此 (cǐ), meaning 'this.' Together, they literally mean 'by this' — referring to something just mentioned or established in context. It signals logical consequence or method, and is common in essays, reports, academic writing, and formal speech.

Unlike spoken alternatives like 所以 or 因此, 以此 emphasizes agency or instrumentality — not just cause-and-effect, but how something is accomplished or justified. It often appears at the beginning of a clause or sentence, followed by a verb or result. While grammatically versatile, it sounds stiff in casual conversation and is rarely used in daily spoken Chinese.