Step 1: Recall what beta decay means.
Beta decay is a radioactive process in which an unstable atomic nucleus changes its composition by emitting a beta particle.
There are two forms of beta decay: beta-minus ($\beta^-$) and beta-plus ($\beta^+$).
Step 2: Focus on beta-minus decay.
In beta-minus decay, a nucleus contains an excess of neutrons compared to protons.
To move toward stability, one neutron inside the nucleus transforms into a proton.
Step 3: Describe the nuclear transformation.
The nuclear reaction for beta-minus decay can be written as:
\[ {}_Z^A X \rightarrow {}_{Z+1}^A Y + e^- + \bar{\nu}_e \]
At the particle level, this corresponds to:
\[ n \rightarrow p + e^- + \bar{\nu}_e \]
Here, a neutron converts into a proton while emitting an electron and an electron antineutrino.
Step 4: Clarify the nature of the emitted particle.
The emitted electron is created inside the nucleus during the decay process.
It is not an orbital electron that was previously surrounding the atom.
This electron is what is referred to as the beta-minus particle.
Step 5: Final conclusion.
The particle emitted in beta-minus decay is:
\[ \boxed{\text{electron}} \]