Nouns

(n)simpleany herbaceous plant having medicinal properties
(n)simpleton, simplea person lacking intelligence or common sense

Adjectives

(a)simplehaving few parts; not complex or complicated or involved“a simple problem”, “simple mechanisms”, “a simple design”, “a simple substance”
(s)elementary, simple, uncomplicated, unproblematiceasy and not involved or complicated“an elementary problem in statistics”, “elementary, my dear Watson”, “a simple game”, “found an uncomplicated solution to the problem”
(s)bare, mere, simpleapart from anything else; without additions or modifications“only the bare facts”, “shocked by the mere idea”, “the simple passage of time was enough”, “the simple truth”
(s)childlike, wide-eyed, round-eyed, dewy-eyed, simpleexhibiting childlike simplicity and credulity“childlike trust”, “dewy-eyed innocence”, “listened in round-eyed wonder”
(s)dim-witted, simple, simple-mindedlacking mental capacity and subtlety
(a)simple, unsubdivided(botany) of leaf shapes; of leaves having no divisions or subdivisions
(s)simpleunornamented“a simple country schoolhouse”, “her black dress — simple to austerity”