Antonym: elective. Similar words: require, requirement, inquire about, inquiry, request, frequent, frequency, frequently. Meaning: [rɪ'kwaɪə(r)d] 
adj. 1. necessary for relief or supply 2. required by rule.
adj. 1. necessary for relief or supply 2. required by rule. 241. Analytical techniques such as atomic absorption spectrophotometry and neutron activation analysis may be required for quantitative determination at such low levels.