'Understanding' in the Bible

for if I pray in an unknown tongue, my spirit doth pray, and my understanding is unfruitful.

What then is it? I will pray with the spirit, and I will pray also with the understanding; I will sing psalms with the spirit, and I will sing psalms also with the understanding;

but in an assembly I wish to speak five words through my understanding, that others also I may instruct, rather than myriads of words in an unknown tongue.

Brethren, become not children in the understanding, but in the evil be ye babes, and in the understanding become ye perfect;