Latin for *a fuller sense of*. The deeper meaning intended by God but not intended by the human author in Biblical exegesis.
Sensus plenior is used by different people in different ways. The most common meaning used in debates among Catholics and later among Evanglicals is the one offered by Raymond Brown:
The sensus plenior is that that additional, deeper meaning, intended by God but not clearly intended by the human author, which is seen to exist in the words of a biblical text (or group of texts, or even a whole book) when they are studied in the light of further revelation or development in the understanding of revelation. Raymond E. Brown, The Sensus Plenior of Sacred Scripture (Baltimore: St. Mary's University, 1955), p 92
Sensus plenior resembles allegory, but those who use it say it is different because there is a strict set of rules.
For more nformation see: What is Sensus Plenior and how does it impact the field of hermeneutics?