**Labelled dose or nominal dose**^{#} | | The mass of drug that is available within the aerosol generator per actuation. This is the dose that is metered. |

**Total emitted dose or delivered dose**^{#} | TED | The mass of drug emitted per actuation that is actually available for inhalation at the mouth. |

**Fine-particle dose** | FPD | The mass of particles <5 μm in size within the total emitted dose. |

**Fine-particle fraction** | FPF | The fine particle dose divided by the total emitted dose. |

**Aerodynamic equivalent diameter** | d_{ae} | The diameter of a fictitious sphere of unit density (1 g·cm^{−3}) that has the same gravitational (settling) velocity in the same gas as the actual particle. |

**Mass median aerodynamic diameter** | d_{ae,μm} or MMAD | The MMAD divides the aerosol size distribution in half. It is the diameter at which 50% of the particles of an aerosol by mass are larger and 50% are smaller. |

**Geometric standard deviation** | σ_{g} or GSD | The GSD measures the dispersion of particle diameter and is defined as the ratio of the median diameter to the diameter at ±1 sd (σ) from the median diameter. In a cumulative distribution plot of the aerodynamic diameter and mass of particles, the GSD is calculated as the ratio of the median diameter to the diameter at 15.9% of the probability scale, or the ratio of the diameter at 84.1% on the probability scale to the median diameter. Aerosols with a GSD ≥1.22 are considered polydisperse. Most therapeutic aerosols are polydisperse and have GSDs in the range of 2–3. |