Definition of oak noun from the Oxford Advanced Learner's Dictionary

oak

noun

əʊk

oʊk

1 [countable, uncountable] (also oak tree [countable]) a large tree that produces small nuts called acorns. Oaks are common in northern countries and can live to be hundreds of years olda gnarled old oak treeforests of oak and pine see also poison oak2 [uncountable] the hard wood of the oak treeoak beamsThis table is made of solid oak.

Idioms

great/tall oaks from little acorns grow

(saying) something large and successful often begins in a very small way