Tolerance is a word describing certain changes in the way an addict reacts to a drug

SUBSTANCE ABUSE DEFINITIONS

Acid (LSD) LSD, also called "acid," is sold in the street in tablets, capsules, or
even liquid form. It is clear and odorless, and is usually taken by mouth. Often
LSD is added to pieces of absorbent paper divided into small decorated squares,
each containing one dose. LSD is a hallucinogen and a very powerful mood-
altering chemical.

Addiction is another way of saying “dependence.” When a person is addicted
to a drug, say for example alcohol, we refer to the condition as “alcohol
dependence.”
Addiction (or dependence) is a syndrome including withdrawal symptoms, tolerance, inability to quit or cut back, and other problems.
Alcohol is a legal, addictive drug that depresses the central nervous system.
Driving while intoxicated is illegal in all states in the US. Even after one drink (1
oz of hard liquor, 1 beer, 1 glass of wine), driving ability is impaired. Alcohol is
cumulatively poisonous, and damages many organs of the body when used
excessively (including the brain, liver, and heart). Chronic, heavy use of alcohol
may lead to irreversible physical and neurological damage.
Alcoholism is another way of saying “alcohol dependence.” A person
diagnosed with alcoholism is therefore addicted to alcohol.

Cocaine is a strong central nervous system stimulant that affects the
distribution of dopamine, a chemical messenger associated with pleasure.
Dopamine part of the brain's reward system and helps create the high that comes
with cocaine consumption. Cocaine usually looks like a white powder used for
sniffing or snorting, injecting, and smoking (in the case of free-base and crack
cocaine). In addition to the desired high, cocaine may produce feelings of
restlessness, irritability, and anxiety, or even mania or psychosis.
Ecstasy (MDMA) is the so-called “party drug," It has both stimulant (like
cocaine) and hallucinogenic (like LSD) effects. Ecstasy is neurotoxic (poisonous
to brain cells), and in high doses it causes a steep increases in body temperature
leading to muscle breakdown, and possible organ failure. Side effects may last
for weeks after use, and including high blood pressure, faintness, confusion,
depression, sleep problems, anxiety, and paranoia.
Heroin is a very addictive drug processed from morphine, a substance
extracted from the seedpod of the Asian poppy plant. Heroin produces a feeling
of euphoria (a "rush") and often a warm flushing of the skin, dry mouth, and
heavy feelings in the arms and legs. After the initial euphoria, the user may go
into an alternately wakeful and drowsy state. Heroin is the second most frequent
cause of drug-related deaths.

Marijuana (weed, or cannabis) is one of the most common drugs of abuse in
Nebraska. Marijuana looks like a dry, shredded green/brown blend of flowers,
stems, seeds, and leaves of a particular hemp plant. It usually is smoked as a
cigarette, pipe, or in blunts, which are cigars that have been emptied of tobacco
and refilled with marijuana. The main active chemical in marijuana is THC (delta-
9-tetrahydrocannabinol), which quickly passes from the lungs into the
bloodstream, and on to organs throughout the body, including the brain.
Some of the short-term effects of marijuana use include problems with memory
and learning; bizarre or distorted perceptions; difficulty in problem solving; loss of
coordination; and increased heart rate.
Methamphetamine (“meth”) is made in illegal laboratories and has a high
potential for abuse and dependence. It is often taken orally, snuffed, or injected.
Methamphetamine hydrochloride, clear crystals resembling ice, can be inhaled
by smoking, and is referred to as "ice," "crystal," and "glass." Use of
methamphetamine produces a fast euphoria, and often, fast addiction. Chronic,
heavy use of methamphetamine can produce a psychotic disorder which is hard
to tell apart from schizophrenia (methamphetamine induced psychosis). The
drug also causes increased heart rate and irreversible damage to blood vessels.
LSD (See Acid)
Over the counter drugs. Many different types of over-the-counter drugs and
other substances can be abused. Just a few examples include:
9 Inhalants (paint thinners, nitrous oxide, model glue, magic marker fluid, spray paints, propane, butane, etc.) 9 Dramamine 9 Mouthwashes 9 Diet aids 9 Cough and cold medications (especially those containing DXM, like Drixoral Cough Liquid Caps, Robitussin AC, Dectuss, Phenergan etc.)
Prescription drugs. Using a prescription drug in a manner other than the
intended prescription constitutes drug abuse. Some of the more commonly
abused prescription drugs are:
9 Pain-relieving narcotics (Percodan, Codeine, Vicodin, 9 Tranquilizers and sedatives (Halcion, Xanax, Ativan, Valium, 9 Muscle relaxants (Soma) 9 Prescription amphetamines (Ritalin, Cylert, Adderall) 9 OxyContin Substance Abuse basically means that a person’s use of substances is
causing problems in life (“failure to fulfill major role obligations at work, school, or
home”). The substance abuser may show lapses in parenting skills, job
functioning, or even legal charges (such as DUI) because of using the substance.
A person diagnosed with substance abuse is not considered to be addicted or dependent (otherwise the diagnosis would be substance dependence).
Substance Dependence is a more advanced problem, accompanied by
certain changes in the way the person relates to the substance. Signs of
dependence include all the signs of abuse plus some additional problems:
9 Experiences withdrawal when not using 9 Seems unable to stop 9 Devotes a lot of time and energy to getting and using 9 Needs more and more to get the same effect (tolerance) 9 Gives up things that used to be important in order to use 9 Compulsions or cravings to keep using

Tolerance is a word describing certain changes in the way an addict reacts to
a drug. A person who develops tolerance needs more and more of the drug to
get the same effect as before.
For example, a person might be able to get a “buzz” after just a couple of beers in the beginning. But as tolerance develops, the person is likely to need to drink more and more to get that buzz.

Source: http://www.nebhands.nebraska.edu/files/Substance%20Abuse%20Definitions.pdf