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The term used to describe the horizontal rows in the periodic table is "periods." Each period indicates a new electron shell being filled with electrons as you move from left to right across the table. This organization reflects the periodicity of element properties, including atomic structure and reactivity, allowing for trends to be observed as one moves across the periods.

Groups, on the other hand, refer to the vertical columns in the periodic table, where elements share similar chemical properties due to having the same number of valence electrons. Families are similar to groups in that they denote collections of elements that exhibit similar characteristics, typically grouped together in columns. Columns highlight similarities among elements, but they do not define the horizontal structure that periods represent.