Alternative analysis (also called alternatives analysis) is one of the tools and techniques for the PMI processes to plan schedule management and to estimate activity resources. It is an analytical technique described in section 6.4.2.2 of the PMBOK, 5th edition.
Alternative analysis is an examination of other ways the project activities may be accomplished. For instance different resources may be substituted. These substitutions may have an impact on other project constraints. Staff with lower skill levels could be used rather than experts. This is likely to affect the time to complete the task. It could also affect the cost in either way -- the lower skilled staff will probably have lower salary rates, but may take enough longer that the cost ends up being higher. Another example would be renting a larger truck -- costing more, but potentially requiring less time and fewer trips.
Related: resources, make-or-buy decisions