A synapse is defined as:

A synapse is indeed defined as a structure that allows a neuron to pass a signal to another cell. This crucial component of the nervous system facilitates communication between neurons, as well as between neurons and various types of target cells, such as muscle or gland cells.

In a synapse, the signaling neuron releases neurotransmitters across the synaptic gap, which then bind to receptors on the receiving cell, leading to the transmission of the signal. This process is essential for everything from reflexes to complex behaviors and cognitive functions.

The other options describe concepts related to synapses but do not accurately define what a synapse is. For instance, options that talk about chemicals involved in signal transmission or insulation detail important functions and substances but do not encapsulate the structural role and the direct action that a synapse performs in passing signals between cells.