This is what it means, that the yield of the olive tree varies from year to year, with a high yield one year and a low one another. I don't know if there is another term for it, but it is basically this.

veceria | WordReference Forums
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Aug 27, 2010 - 4 posts - ‎3 authors
... oliva y no encuentro la traducción al inglés de la palabra Vecería:. ... The term I've always seen is "alternate bearing": http://www.google.com.

"The olive tree (Olea europaea L.) is widely known for its strong tendency for alternate bearing, which severely affects the fruit yield from year to year.Mar 28, 2013"

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Note added at 35 mins (2017-04-08 07:16:38 GMT)
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Explanation in Spanish:

"Hoy toca hablar de la vecería, ese fenómeno producido en los olivos que consiste en alternar grandes producciones de fruto, olivas o aceitunas, con otros años en los que las produciones son considerablemente más bajas, siendo este un fenómeno bien conocido por los agricultores dedicados a este sector, sufriendo en mucho menor medida los efectos de la vecería otros frutales como manzanos, cítricos, aguacates, mangos, etc…"
https://cienciaycampo.wordpress.com/2011/05/15/veceria-en-el...

"The intensity of pruning should also take the crop load into consideration. It is especially important because of the alternate bearing habit of the olive tree. In heavy cropping years, the growth of the tree is reduced, so pruning should be limited to the elimination of water sprouts and weak shoots. Alternatively, trees should be pruned more severely after years of low yields."
http://www.oliveoilsource.com/page/olive-tree-pruning