The act or result of addition; a sum.
"After carefully summing all the receipts from last month, she found that her expenses were slightly higher than expected."
present participle of sum
"After hours of calculations, she was finally summing up all her expenses for the month."
Alternative form of something
"In the old copy of the recipe, she noted that "sugar in powder form" was an alternative spelling for sugarum, which is actually a mistaken use of summing as an alternative form of something."