Solo 5500 glossary
Plain-English definitions for IRS Form 5500-EZ and Solo 401(k) terminology, with links back to the official IRS or DOL source when available.
Form 5500-EZ
IRS annual return for one-participant retirement plans (owner-only Solo 401(k), partner-spouse plan, or foreign plan).
Form 5500-SF
DOL/IRS short-form annual return for eligible small employee benefit plans (under 100 participants).
EFAST2
Department of Labor's electronic filing system for Form 5500-series returns.
IFILE
DOL's free browser-based filer used to submit Form 5500-series returns through EFAST2.
One-participant plan
Retirement plan that covers only the business owner(s) and their spouse(s), or only partners and their spouses.
Solo 401(k)
401(k) retirement plan covering only a self-employed business owner and optionally their spouse.
Common-law employee
A worker the IRS classifies as an employee under the common-law right-of-control test, distinct from independent contractors.
Plan sponsor
Employer or business entity that establishes and maintains the retirement plan.
Plan administrator
Person or entity designated under the plan document to manage day-to-day operations and required filings.
Plan year
12-month period for which plan records are kept and Form 5500-EZ is filed.
Plan number (PN)
Three-digit identifier assigned by the sponsor to each plan they sponsor.
EIN
Employer Identification Number — 9-digit IRS-issued tax ID used on Form 5500-EZ.
Form 5558
IRS Application for Extension of Time To File Certain Employee Plan Returns — grants 2½-month extension for Form 5500-EZ.
Controlled group
Two or more businesses connected through common ownership and treated as one employer for retirement plan testing.
Affiliated service group (ASG)
Group of related businesses providing services to one another, treated as one employer under IRC Section 414(m).
Participant loan
Loan from a Solo 401(k) plan to its participant, capped at the lesser of $50,000 or 50% of vested balance.
Deemed distribution
Treated-as-distributed amount when a participant loan defaults under IRC Section 72(p).
DFVCP
DOL Delinquent Filer Voluntary Compliance Program — reduced-penalty cure for late Form 5500/SF filings (does NOT cover Form 5500-EZ).
IRS Late Filer Penalty Relief Program
Rev. Proc. 2015-32 program that lets one-participant plan filers cure late Form 5500-EZ at $500/return capped at $1,500/submission.
Form 5500-EZ deadline
Last day of the 7th month after the plan year ends — July 31 for calendar-year plans.
$250,000 asset threshold
Combined one-participant plan asset level above which Form 5500-EZ filing becomes required.
Final plan year
Plan year in which all plan assets are distributed or transferred — requires a final Form 5500-EZ even below $250,000.
10-return e-filing threshold
Sponsors required to file 10+ IRS returns in the calendar year must file Form 5500-EZ electronically through EFAST2 (effective January 1, 2024).
Form 8955-SSA
IRS Annual Registration Statement Identifying Separated Participants With Deferred Vested Benefits.
Required Minimum Distribution (RMD)
Annual minimum withdrawal required from Solo 401(k) starting at age 73 (SECURE 2.0 Act).
SECURE 2.0 Act
2022 federal law making 90+ changes to retirement plan rules including RMD age, Roth Solo 401(k), and catch-up rules.
Roth Solo 401(k)
Solo 401(k) sub-account funded with after-tax employee deferrals (and now employer contributions per SECURE 2.0).
Catch-up contribution
Extra annual deferral allowed for participants age 50+ ($7,500 in 2025) or age 60-63 ($11,250 in 2025 per SECURE 2.0).
Pre-approved plan
Solo 401(k) document drafted by a sponsor and pre-approved by the IRS via opinion letter — adopting employers rely on the letter.
Opinion letter
IRS letter approving the form of a pre-approved retirement plan document for the current 6-year cycle.