= Resonance hybrid
A <resonance hybrid> is the combined electronic description expressed using several <resonance structures>. The word does not describe a mixture of separate <molecules> or <ions>: a <nitrate ion> has one <electron density>, rather than three species in <chemical equilibrium>. Equal weighting is appropriate for the equivalent <nitrate> contributors in the elementary model; the unequal <carbon dioxide> contributors illustrate why it is not a general rule. See https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance[Chemistry 2e, 7.4: resonance hybrids do not fluctuate between forms].
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