The total interval of geologic time which is not represented by strata at a specified position along a given stratigraphic surface. This could be a response to non deposition or erosion, though most think of this as a time of non deposition.  If the hiatus encompasses a measurable interval of geologic time, the stratigraphic surface is an unconformity (Mitchum, 1977).
Wheeler (1964) identified “missing” time is a lacuna of two parts:

• Hiatus - time value of non-deposition and erosion degradation
• Vacuity - time value of previous deposits that were removed by erosion

He saw an hiatus as represented by a single surface with three separate domains of time

• Non-deposition
• Erosion
• Material that has been removed

References
Mitchum Jr., R. M., (1977), Seismic Stratigraphy and Global Changes of Sea Level: Part 11. Glossary of Terms used in Seismic Stratigraphy: Section 2. Application of Seismic Reflection Configuration to Stratigraphic Interpretation, Memoir 26 Pages 205 - 212. )
Wheeler, H.E. 1958, Time stratigraphy. AAPG Bulletin, Vol. 42, p 1047-1063. Wheeler, H.E. 1964, Baselevel, Lithosphere Surface, and Time-stratigraphy Geological Society of America Bulletin, July 1964, v. 75, no. 7, p. 599-610