Sentence:  The widow had but only one, / A puny and decrepid son; / But day and night, / Though fretful oft, and weak, and small, / A loving child, he was her all— / The <WSD>widow's mite</WSD>. 
Meaning of the word marked with <WSD> tags:  A very small gift or donation which, however, represents a great sacrifice on the part of the giver.