| Term | description |
| ACI | Adjacent Channel Interface. ACI is interference caused by extraneous power from a signal in an adjacent channel. |
| Active Tag | An RFID tag that has a transmitter to send back information rather than reflecting back a signal from the reader like passive tags do. Most active tags use batteries as a partial or complete source of power to transmit a signal back tot he reader. Active tags can be read from long distances, but they're much more expensive than passive tags. They're mostly used for tracking expensive items over long ranges. |
| AFC | Automatic Frequency Control. A device or circuit that maintains the frequency of an oscillator within the specified limits with respect to a reference frequency. |
| AGC | Automatic Gain Control. A device or circuit that maintains the gain of a circuit or amplifier by comparing it with a known reference. |
| AGC Threshold | The level of input current at which the AGC circuit becomes active. |
| AGC Time Constant | The amount of time it takes to achieve the required AGC level; also, the amount of time it takes to recover from AGC. |
| Air Interface Protocol | The rules that govern how RFID tags and RFID readers communicate. |
| AM | Amplitude Modulation. A form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of a modulating signal. |
| Amplitude | The maximum absolute value of a periodic curve measured along its vertical axis (the height of a wave, in layman's terms). |
| Antenna | A structure that is intended to radiate electromagnetic waves into or collect electromagnetic energy from space. |
| ASIC | Application Specific Integration Circuit. An integrated circuit (IC) customized for a particular use, rather than intended for general-purpose use. |
| ASSP | Application Specific Standard Product. An integrated circuit that implements a specific function that appeals to a wide market, as opposed to ASICs that are designed for one customer. |
| Attenuation | The degree to which an input signal is reduced in amplitude at the output of a circuit or device. Measured in dB. |
| Auto-ID Center | A non-profit collaboration between private companies and academia that pioneered the development of an Internet-like infrastructure for tracking goods globally through the use of RFID tags. |
| Automatic Identification | A broad term that covers methods of collecting data and entering it directly into computer systems without human involvement. Technologies normally considered part of auto-ID include bar codes, biometrics, RFID and voice recognition. |
| ACPR | Adjacent Channel Power Ratio. The ratio of unwanted power in the adjacent channel relative to the wanted channel power. |
| Active Bias | A circuit that employs feedback to maintain bias level. Often a constant current circuit is used. |
| ADC | Analogue to Digital Converter. An electronic circuit that converts an analogue signal into a binary number. The reverse operation is formed by a DAC. |
| Ampere (A) | A unit of electric current equal to one coulomb (C) of charge per second. |
| AMPS | Advanced Mobile Phone System. An analog mobile phone system standard introduced in the Americas in 1984. |