, this means made with the tip
) of the tongue, as opposed to laminal
(made with the blade
) and dorsal
(made with the back of the tongue).
The apex is the active articulator, and the bit of the mouth it touches is called the passive articulator. Apical articulation normally occurs with alveolar sounds, where the apex touches the alveolar ridge just behind the upper teeth. Hence these are called apico-alveolar. Neighbouring points of articulation are usually laminal.
It is possible to reverse this, and make lamino-alveolar and apico-dental sounds, but I believe they are not so common in language. This correspondence is a generalization, and varies betweeen languages and even between speakers. It is possible for both the blade and the tip to be in use, giving apico-laminal sounds.
One odd contrast is in Basque, where there are two distinct voiceless sibilants s (written in the Basque alphabet as s and z), both of them alveolar, but the duller one (s as in Spanish) is apico-alveolar, whereas the sharper one (written z, but pronounced like the s of French or Russian) is lamino-alveolar. I find this distinction quite difficult to make consistently. There are also corresponding affricates ts and tz.
The retroflex sounds of Indian languages are apical, specifically apico-postalveolar, with additionally the tongue turned back (retroflexed).
The difference between the tr and ch sounds of English (train, chain) might be (and I am speculating here - it is unclear from my reading and is apparently a disputed topic among phoneticians) that while both are postalveolar, tr is apical while ch is laminal.
In phonology apical sounds are distinguished as having the feature [-distributed], whereas laminals are [+distributed].
This word, by the way, may be pronounced either with the long A of apex, or shortened as in apt.