a recruitment selection event that ranges from a couple of hours to a couple of days
a standardised selection method which uses a variety of different tests, interviews and exercises to evaluate a candidate's potential performance in a particular post
"The preferred organisation for a breast screening programme is to have a single assessment centre with all members of the assessment team being based at the same site as the assessment centre. However, this is not always possible in practice. The need to ensure that a programme serves a large enough population to enable it to be properly quality assured and achieve the NHSBSP minimum standards may mean that assessment will take place at several sites, or that referrals for surgical assessment and treatment are made to breast surgeons at one or more peripheral hospitals." -- NHSBSP 52.
This refers to a selection process that may span one or more days and that incorporates multiple assessment measures. Because you are not just relying on any one selection tool, such as the interview, the results tend to be the most predictive in terms of selecting quality candidates; but only if you know how to professionally apply the Assessment Centre approach.
a popular assessment technique which comprises a range of assessment methodologies. The defining feature of an assessment centre is some simulation of the task. They are usually demanding events which can last from half a day to three or more days. The simulation of the job gives individuals a degree of insight into what is involved in the role. Assessment centres although widely used are also seen as problematic. They often have very poor construct validities—it is not clear what they are actually measuring and this reduces their ability to predict future job performance accurately.