The opposite of an acid is a base, and that's exactly what an antacid is. Antacids make you feel better by increasing the pH balance in your stomach. The pH system is a scale for measuring the acidity or alkalinity of a given environment (in this case, your stomach). The scale goes from 0 to 14. A pH of 7 is neutral. Below 7 is acid. Above 7 is alkaline.
Economy is the large set of inter-related economic production and consumption activities which aid in determining how scarce resources are allocated.
A market economy is one where the allocation of resources and the trading of goods and services are through the decentralized decisions of many firms and households. Prices and the willingness of the market to pay those prices determines economic output, which, in turn, determines the allocation of resources.