CBSE Class 10 Answered

Apical dominance is the inhibition of the growth of lateral buds by the terminal or apical bud of a plant shoot.

 So the main central stem of the plant is dominant over (i.e., grows more strongly than) other side stems. On a branch the main stem of the branch is further dominant over its own side branchlets.

The apical bud or tip produces the growth hormone auxin, which not only promotes cell division, but also diffuses downwards and inhibits the development of lateral bud growth which would otherwise compete with the apical tip for light and nutrients. Removing the apical tip and its suppressive hormone, allows the lower dormant lateral buds to develop, and the buds between the leaf stalk and stem produce new shoots which compete to become the lead growth.

Synaptic cleft is the minute gap between presynaptic cell and postsynaptic cell in a chemical synapse, across which the neurotransmitters diffuse into.
This gap acts as a site where neurotransmitters from presynaptic cell (e.g. neuron) are released into by exocytosis and diffuses across to bind with the receptors in the cell membrane of postsynaptic cell. This gap is only a minute space allowing the concentration of neurotransmitters to be raised and lowered rapidly.