Acidosis	an excessively acid condition of the body fluids or tissues.
Affective states	emotions and other feelings in animals that are experienced as pleasant or unpleasant.
Amygdala	a structure in the limbic system of the brain that is linked to emotions and aggression.
Anoxia	absence of oxygen.
Anxiogenic	anxiety causing.
Anxiolytic	anxiety inhibiting.
Approach-avoidance test	a test that uses a desirable reward (e.g., a food treat) to evaluate whether an inhaled agent is aversive and, if so, to what extent. Animals choose whether or not to access the reward in the presence of a range of concentrations of the agent, which provides a way of testing their motivation for the treat against their motivation to avoid the agent. For example, rats and mice always choose to avoid exposure to argon or CO2 even when they are food deprived and a preferred treat is present, indicating that these gases are highly aversive.
Aversion	avoidance of a stimulus, situation or behaviour.
Aversive	causing avoidance of a stimulus, situation or behaviour.
Aversion-avoidance test	a test that uses an animal’s motivation to avoid an unpleasant stimulus to evaluate its level of aversion to an inhaled agent. One commonly used paradigm is for rodents to choose between a preferred, dark chamber that is filling with a gas and an aversively brightly-lit chamber. Rats and mice always prefer the bright chamber over one filling with CO2, but some choose the dark chamber filling with anaesthetic until they lose consciousness.
Aversiveness	see aversion, aversive.
Chemotaxic	the movement of an organism in response to a chemical stimulus.
Conditioned place aversion	a commonly used technique to evaluate aversion to a specific environment that has been associated with a negative reward.
Conditioned place preference	a commonly used technique to evaluate preferences for stimuli that have been associated with a positive reward. In general, this procedure involves several trials where the animal is presented with the positive stimulus (e.g., food) paired with placement in a distinct environment containing various cues (e.g., tactile, visual, and/or olfactory). When later tested in the absence of the stimulus, approaches to and/or the amount of time spent in the compartments previously associated with the positive stimulus serve as indicators of preference.
Distress	a strongly negative emotional experience, associated with stress of such magnitude or duration that significant changes in biological function would be necessary for the animal to survive if it could not easily escape the stressor.
Dyspnoea	a subjective experience of breathing discomfort; “air hunger”.
Electroencephalogram (EEG)	a recording of the electrical activity of the brain, typically representing the activity of the cerebral cortex in mammals.
Electromyogram (EMG)	a recording of the electrical activity produced by skeletal muscles.
Eustress	“good” or “beneficial” stress, occurring in response to positive events, but which may give rise to the same physiological indicators of stress as negative events.
Euthanasia	frequently used to describe the process of killing laboratory animals, euthanasia means a “good death”, which the AVMA [47] defines as a death that occurs with “minimal pain and distress”. The term is also often applied to both humans and animals in the context of ending suffering [64]. Given this, the generic use of the term “euthanasia” for laboratory animals is questionable for two reasons. First, the humaneness of some methods used to kill laboratory animals remains in doubt, such that “killing” is often considered a better description of the processes used to end the lives of laboratory animals. Second, animals are sometimes killed for reasons other than ending their suffering (e.g., if an experiment requires their tissues or if they are surplus to requirements). The term “humane killing” is therefore generally more appropriate than euthanasia in relation to laboratory animals.
HPA axis	hypothalamic–pituitary–adrenal axis; the system of feedback interactions among the hypothalamus, pituitary gland, and adrenal glands that controls reactions to stress and regulates many body processes.
Humane endpoint	one or more predetermined physiological or behavioural signs that define the point at which an experimental animal’s pain and/or distress will be terminated, minimized or reduced by taking actions such as killing the animal, terminating a painful procedure or giving treatment to relieve pain.
Humaneness	characterised by concern or compassion for others; in the context of methods of humane killing, that which causes little or no pain or distress.
Hypercarbic	containing excess CO2.
Hypoxia	deficiency in the amount of oxygen reaching the tissues.
Inhaled agent	a substance (gas or vapour) which is administered via the inhalation route.
Isoelectric	having or involving no net electric charge or difference in electrical potential; in EEG a signal representing no on-going brain activity.
Monosynaptic connections	passing through a single synapse (junction between two nerve cells).
Neonatal	relating to the immediate period after birth. The age of neonatal rodents has been defined as up to and including day 10, where the day of birth is day 1 [56].
Nociceptor	a sensory receptor (nerve) for painful stimuli.
Normoxic	containing the normal atmospheric concentration of oxygen (~21%).
Olfactory bulb	a neural structure of the vertebrate forebrain involved in olfaction or the sense of smell.
Pavlovian association	an association between a previously unrelated neutral stimulus (e.g., bell) and another stimulus (e.g., food) that reliably elicits a reaction (e.g., salivation).
REM	REM (rapid eye movement) sleep.
Visually evoked potential (VEP)	electrophysiological responses to stimulation by either patterned or unpatterned visual stimuli.