Q.Define "antibiotic" and explain, at a general level, why substances produced by one microorganism can inhibit or kill another.

[!TLDR] An antibiotic is a substance made by one microbe that inhibits/kills another -- a natural outcome of microbial competition for resources.

[!ANSWER] An antibiotic is a chemical substance produced by one microorganism capable of inhibiting or killing another; this occurs because microbes naturally compete for the same limited nutrients and space, and producing such an inhibitory chemical is a competitive advantage for the producer.

An antibiotic is defined as a chemical substance, produced by one microorganism, that is capable of inhibiting the growth of, or directly killing, another microorganism. This ability is, at root, an outcome of natural microbial competition: in any given environment, countless different species of bacteria and fungi are constantly competing for the same limited nutrients and physical space. Producing a chemical that suppresses a rival organism's growth in the immediate vicinity is, for the producing organism, a direct competitive advantage -- it clears away competitors, leaving more resources available for the producer itself. Antibiotics are, in this sense, naturally evolved chemical weapons that humans have learned to isolate and repurpose as medicines to treat bacterial infections.

[!ANSWER] An antibiotic is a microbially produced substance that inhibits or kills another microbe, an ability that evolved because it gives the producing organism a competitive advantage in the fight for shared resources.

Give the formal definition first, then explain the underlying evolutionary/ecological logic (why such an ability would evolve at all) -- competition for resources.

A common gap is defining 'antibiotic' correctly but not explaining WHY such a substance would exist in nature in the first place -- the competitive-advantage reasoning is the deeper part of the question.