As a Noun: 1. The act of putting one person or thing in the place of another person or thing. English = substitution, replacement. 2. The person or thing that takes the place of or performs the function of a second person or thing. English = substitute, replacement. 3. In algebra, the act of putting numbers where the letters (such as a, b, x, y, etc.) are in an equation or a formula. English = substitution. As a Verb or Adjective: 1. For one person or thing to take the place of or perform the function of a second person or thing. English = substitute, replace. Idiomatic English = put in the place of. 2. To use one thing instead of a second thing. English = substitute, replace. 3. In algebra, to put numbers where the letters (such as a, b, x, y, etc.) are in an equation or a formula. English = substitute. As Modifier: 1. Used between the signs for two things or the names of two people, between two actions, or between two times, to mean the first happens rather than the second. English = instead of, in the place of. 2. Used at the beginning of a sentence to mean the action or the thing discussed in the sentence happens rather than something else which has just recently been talked about. English = instead.