The term 'SERIES-ENDING ABBR.' refers to abbreviations that are used to signify the end of a series. The two most common abbreviations for this purpose are ETAL (4 letters) and ETC (3 letters). ETAL stands for 'et alii' which is Latin for 'and others', while ETC stands for 'et cetera' which is Latin for 'and so forth'. These abbreviations are used to indicate that there are more items in the series than what is listed, and they are commonly used in academic writing.