The amount of work or labor (in hours or work days) required to complete a task. Effort is used to establish the labor costs associated with a project. Effort is in contrast to duration, which is the span of time over which the effort takes place.
(Complete Idiot’s Guide to Project Management)
Effort – The number of labor units necessary to complete the work . Effort is usually expressed in staff hours, staff days or staff weeks and should not be confused with duration .(Wideman Comparative Glossary of Common Project Management Terms v2.1 )