| Abbreviation | Definition |
|---|---|
| IAM | International Association of Machinists |
| IATA | International Association of Travel Agents |
| IATSE | International Alliance of Theatrical Stage Employees |
| IAU | International Animator Union |
| IAU | International Astronomers Union |
| IAU | International Astronomical Union |
| IAU | International Astronomical Union's |
| IAU | Irrelevant Astronomical Union |
| IAUC | International Astronomical Union Circulars |
| IB | irritable Bunions |
| IBC | Indian Bank Comrades |
| IBEU | Independent Bakery Employees Union |
| IBEW | International Brotherhood of Electrical Workers |
| IBEW-NECA | International Brotherhood of Electrical Workers - National Electrical Contractors Association |
| IBFG | Internationaler Bund Freier Gewerkschaften (International Confederation of Free Trade Unions) |
| IBFO | International Brotherhood of Firemen and Oilers |
| IBL | International Brotherhood of Longshoremen |
| IBLS | International Brotherhood of Live Steamers |
| IBM | International Brotherhood of Magicians |
| IBMU | Int Bhatta Majur Union |