The IES defines illumination engineering terms in ANSI/IES LS-1-22, is available here online. You can search terms by their first letter using the categories listed in the Categories window, by the first letter of the term in the alphabetical list below, or by typing the term in the search window. You can find the Introduction material for LS-1-22 on these pages:
Copyright, Disclaimer, and American National Standards Statement
Foreword, Nomenclature Committee members, Introduction, and Scope

- Abbe’s law
- absolute luminance threshold
- absorptance, α
- absorption
- accent lighting
- acceptance half-angle
- accommodation
- ace
- actinic
- action spectrum
- adaptation
- adaptive color shift
- adaptive lighting
- adaptive subdivision
- adverse weather lamp
- aerodrome beacon
- aeronautical beacon
- aeronautical ground light
- aeronautical light
- afterimage
- aging
- air mass (optical air mass)
- aircraft aeronautical light
- airway beacon
- Allard’s law
- altitude angle
- amalgam
- ambient lighting
- analog multiplex, AMX
- anchor light
- angle-of-approach lights
- angle of collimation
- Annex A – Physical Receptors and Devices
- annual sunlight exposure (ASE)
- anti-collision light
- aphakic hazard function
- apostilb
- approach-light beacon
- approach lights
- arc
- arc discharge
- arc lamp
- area lighting (television, film, theater)
- artificial pupil
- asbestos tapoff
- astronomical time switch
- atmospheric transmissivity
- automated fixture
- auxiliary equipment
- average luminance
- aversion response
- azimuth