as·sump·tion 
/əˈsʌmpʃən/ noun
plural as·sump·tions /əˈsʌmpʃən/ noun 1 [count] : something that is believed to be true or probably true but that is not known to be true : something that is assumed
▪ I made the assumption that he was coming, so I was surprised when he didn't show up. ▪ He will come home tomorrow. At least, that's my assumption. ▪ Many scientific assumptions about Mars were wrong. ▪ We are operating on/under/with the assumption that the loan will be approved. [=we are assuming that the loan will be approved; we are behaving as though we know that the loan will be approved] ▪ I'm telling you our arrival time on the assumption that you will check to see whether or not our flight is on time before you come to the airport. ▪ Her plan is based on the underlying assumption that the economy will improve in the near future. 2 [noncount] : the act of assuming something: such as a : the act of beginning a role, duty, etc.
▪ her assumption of the presidency
b : the act of taking or beginning to have power, control, etc., in a job or situation
▪ the assumption of control/authority
c : the act of accepting a responsibility, debt, etc.
▪ the buyer's assumption of debt
▪ I made the assumption that he was coming, so I was surprised when he didn't show up. ▪ He will come home tomorrow. At least, that's my assumption. ▪ Many scientific assumptions about Mars were wrong. ▪ We are operating on/under/with the assumption that the loan will be approved. [=we are assuming that the loan will be approved; we are behaving as though we know that the loan will be approved] ▪ I'm telling you our arrival time on the assumption that you will check to see whether or not our flight is on time before you come to the airport. ▪ Her plan is based on the underlying assumption that the economy will improve in the near future.
▪ her assumption of the presidency
b : the act of taking or beginning to have power, control, etc., in a job or situation
▪ the assumption of control/authority
c : the act of accepting a responsibility, debt, etc.
▪ the buyer's assumption of debt
