Dictionary.com
definitions

disk flower

noun Botany.

one of a number of small tubular flowers composing the disk of certain composite plants.
Compare ray flower.

Origin of disk flower

First recorded in 1865–70
Also called disk floret.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
disk flower in Science

disk flower

Any of the tiny tubular flowers in the central portion of the capitulum or flower head of certain plants of the composite family (Asteraceae or Compositae). In many composite flowers, such as the daisy and sunflower, the disk flowers are surrounded by flattened ray flowers.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.