A subapical consonant is a consonant made by contact with the underside of the tip of the tongue. The only common subapical articulations are in the postalveolar to palatal region, which are called "retroflex".
Most so-called retroflex consonants are more properly called apical. True subapical retroflexes are found in the Dravidian languages of southern India.
Occasionally the term "sublaminal" is used for subapical. However, that term might be better used for sounds pronounced between the underside of the tongue and the floor of the mouth, such as sucking-teeth.
References
Subapical consonant Wikipedia(Text) CC BY-SA