(physics and chemistry) the smallest component of an element having the chemical properties of the element
"After years of splitting uranium atoms, the scientists finally achieved a controlled nuclear chain reaction in their laboratory."
The smallest possible amount of matter which still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons.
"Scientists realized that splitting an atom releases enough energy to power entire cities."
In plain English: An atom is the tiniest piece of an element that still keeps all its special properties.
"Scientists use an atom to explain how matter is built from tiny particles."
Usage: Use "atom" to refer to the fundamental unit of a chemical element that cannot be broken down further while retaining its properties. Do not use it to describe an individual person or a tiny speck unless you are deliberately using a metaphorical sense.
The word "atom" comes from the Ancient Greek átomos, meaning "indivisible," which was formed by combining a prefix for "not" with a verb meaning "to cut." It entered English via Middle French and Latin to describe the smallest unit of matter that was historically believed to be impossible to divide.