These words are related but might not refer to the same exact thing. For example if you check the Wikipedia entry for Authoritarianism you will find this entry:
Authoritarianism is a form of social
organization characterized by
submission to authority. It is opposed
to individualism and democracy. In
politics, an authoritarian government
is one in which political power is
concentrated in a leader or leaders,
typically unelected by the people, who
possess exclusive, unaccountable, and
arbitrary power. Authoritarianism
differs from totalitarianism in that
social and economic institutions exist
that are not under the government's
control.
so immediately you see the distinction between authoritarianism vs. totalitarianism. If you now go to the Wikipedia entry for Totalitarianism you will find this entry:
Totalitarianism (or totalitarian rule)
is a political system where the state,
usually under the power of a single
political person, faction, or class,
recognizes no limits to its authority
and strives to regulate every aspect
of public and private life wherever
feasible.Totalitarianism is
usually characterized by the
coincidence of authoritarianism (where
ordinary citizens have less
significant share in state
decision-making) and ideology (a
pervasive scheme of values promulgated
by institutional means to direct most
if not all aspects of public and of public and private life).
We see here that Totalitarianism is characterized by Authoritarianism AND ideology.
If we now check Despotism vs. Dictatorship we find that for Despotism is
a form of government in which a single
entity, called the despot, rules with
absolute power. That entity may be an
individual, as in an autocracy, or it
may be a group, as in an oligarchy.
The word despotism means to "rule in
the fashion of a despot" and should
not be confused with "despot", an
individual.
the key word here is "single entity" which can actually be an individual OR a group. Under dictatorship we find this entry:
A dictatorship is defined as an
autocratic form of government in which
the government is ruled by an
individual, the dictator.
also we find the distinction between dictatorships and totalitarian rule (with reference to authoritarianism:
For some scholars, a dictatorship is a
form of government that has the power
to govern without consent of those
being governed (similar to
authoritarianism), while
totalitarianism describes a state that
regulates nearly every aspect of
public and private behavior of the
people. In other words, dictatorship
concerns the source of the governing
power (where the power comes from) and
totalitarianism concerns the scope of
the governing power (what is the
government). In this sense,
dictatorship (government without
people's consent) is a contrast to
democracy (government whose power
comes from people) and totalitarianism
(government controls every aspect of
people's life) opposes pluralism
(government allows multiple lifestyles
and opinions). Though the definitions
of the terms differ, they are related
in reality as most of the dictatorship
states tend to show totalitarian
characteristics. When governments'
power does not come from the people,
their power is not limited and tend to
expand their scope of power to control
every aspect of people's life.
For autocracy we find this entry:
An autocracy is a form of government
in which one person possesses
unlimited power.1 An autocrat is a
person (such as a monarch) ruling with
unlimited authority.
a distinction is made between autocratic rule vs. miliatry dictatorships with some reference also to totalitarian rule:
Autocracy and totalitarianism are
related concepts. Autocracy is defined
by one individual having unlimited
legislative and executive power, while
totalitarianism extends to regulating
every aspect of public and private
life. Totalitarianism does not imply a
single ruler, but extends to include
absolute rule by any faction or class
of elites who recognize no limit to
their authority. Autocracy differs
from military dictatorship, as these
often take the form of "collective
presidencies" such as the South
American juntas. However, an autocracy
may be totalitarian or be a military
dictatorship.
I hope this helps! :)
Best Answer
I have not seen the word "instable" being used often. The word "instability" exists, though. Funnily, the word "unstability" does not exist.
And even in engineering, the same two words are used - "unstable sorting algorithm", "unstable equilibrium", and as you said, "aircraft instability".
Instability is just the noun form of unstable.
EDIT from comments: The word "unstability" does exist, apparently, but is rarely used. I personally have never seen it. Even the spell check in Firefox marks it as a spelling mistake and suggests "instability" instead.