brought all the way to its proper goal: whole, complete, and lacking nothing it should have
This word is built on the idea of a goal reached, an end fully arrived at. Something is teleios when it has become everything it was meant to be, with no part missing. Greek used it for a grown ox, a ratified contract, a sacrifice without blemish, an adult as opposed to a child. Across all of these the thread is the same: the thing is fully there, finished, entire.
The New Testament inherits this word loaded with extra weight from the Greek Old Testament, where it often translated Hebrew words for being whole and wholly devoted to God. So its uses fan out along a few related lines rather than splitting into unrelated meanings. When the word is set against the word for infant, it means grown-up, mature. When it describes a thing such as a gift, a law, a work, or God's will, it means complete, whole, lacking nothing. When it describes a person's heart before God, it leans toward being undivided and wholly committed.
These senses are not separate words but one idea seen from different angles, and the surrounding sentence decides which angle is in view. The danger is to read the English word perfect, with its sense of flawless or sinless, back into every occurrence. That is almost never what the writers were asking for. They were asking people to be all there: grown up, undivided, complete in character, carried through to the goal God had in mind.