She was unwell, and therefore could not attend the meeting.
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是以 is a classical/literary Chinese expression equivalent to 因此 or 所以. It is rarely used in everyday spoken Chinese; you will mainly encounter it in formal writing, historical texts, or literature.
grammar
In classical Chinese, 是 functions as a demonstrative pronoun ('this') and 以 means 'because of / therefore' - literally 'because of this'. The full phrase is an inversion of 以是 used in ancient texts.
