Last week's word of the week was not a sciencey word, but a fun word, because why not? But this word of the week is one I use ALL the time, and one that I always worry people don't know about. And your brain is full of them, so it seems relatively important to know what they are.
Neurotransmitter: These are chemicals that are present in your brain, and serve a chemical messengers between neurons. Neurons do not actually touch each other, instead there is a little gap between one neuron and the next (called a synapse), and neurotransmitters are released from one neuron to the next, to keep a signal going.
An important note: neurotransmitters are released from inside the first neuron (from little bubbles called vesicles which bleb on to the outer membrane of the neuron and release their chemicals out into the space), but they do NOT go into the next neuron. Instead they hit the next neuron and bind to receptors on the cell surface, which change the inside of the target neuron to pass the message on.
Neurotransmitters can cause the next neuron to have an action potential and an excitatory effect, or they can cause that neuron to shut down temporarily, acting in an inhibitory manner. It all depends both on the type of neurotransmitter, and particularly on the RECEPTOR the neurotransmitter binds to. Examples of neurotransmitters include things like acetylcholine, GABA, glutamate, dopamine, serotonin, and norepinephrine (among others). But the chemical itself is merely one molecule, which is released from one neuron and binds to receptors on another. It is then taken up by transporters or degraded by enzymes.
EDIT: I would like to include a note by Dr. Zen (included in a comment below) about the difference between a neurotransmitter and a neuromodulator, which is a very fine one and worth thinking about.
A chemical that has fast, short-lived effect on a neuron is acting as a neurotransmitter. Neurotransmitters bind to receptors that are doorways in the neuron to let electrical current flow.
A chemical that has a slow, long-lasting effect on a neuron is acting as a neuromodulator. Neuromodulators bind to receptors that trigger events inside the cell, but those receptors don’t act as doorways for currents themselves.
The same chemical can be a neurotransmitter in one location and a neuromodulator in another. The same chemical can be acting as both a neurotransmitter AND a neuromodulator on the same neuron, if that neuron has two different kinds of receptors for that chemical on it.