is,

Worshipfully and solemnly coming before the Lord in loving adoration, confession of sins, supplication for mercy, intercession for blessings upon others, and thanksgiving for blessings received.

A prayer can be as short as asking for one single thing, or as long as doing all of the above.

Be careful for nothing; but in every thing by prayer and supplication with thanksgiving let your requests be made known unto God. And the peace of God, which passeth all understanding, shall keep your hearts and minds through Christ Jesus.
Philippians 4:6-7