There seems to be some confusion over the correct definitions of "psalm" and "hymn."
[It] is difficult to decide to what degree, if at all, a distinction ... is made by the ... terms, psalms, hymns ... (newadvent.org)
"Psalm" is usually defined as "a sacred song or hymn of worship or praise," while a "hymn" is usually defined as "a sacred song of praise." The one distinction gleaned from the usual definitions is that a hymn is a category of psalm, and this seems to be a correct though limited distinction.
According to "The Psalms of David" offered by International Fellowship of Christians and Jews, a "psalm" is style of poetry. An analysis of the poetic genre of psalm will show that "psalm" is incompletely defined as "a sacred song or hymn of worship or praise."
A psalm is a poetic genre having a distinct meter that is limited by words per line: there are only two to four words per line in a Hebrew psalm, and each word is accented. This limits the other features of meter—rhythm and feet—within strict bounds. The most common meter is trimeter—3 accents—for two couplet lines, 3 accents + 3 accents in two lines. Variations on the two-line couplet trimeter employ 3 accents and/or 2 accents for 3 + 2 and 2 + 2. A three-line trimeter triplet also exists: 3 accents + 3 accents + 3 accents in three lines.
Parallelism of repeated or contrasted thoughts is a further component of the structure of a psalm. Kinds of parallelism are:
antithetic for contrasted thoughts
synthetic for complemented, supplemented or completed thoughts
climactic or stair-like for repetition joined with addition (moves the thought ahead in steps) ("The Psalms of David")
A "hymn" is identified by BibleHub.com as any of a number of types of psalm (i.e., the style of poem). BibleHub identifies six types of hymn written as psalms. This illustrates that "hymn" is inadequately defined as "a sacred song of praise":
- Hymns of praise
- National hymns
- Temple hymns, which are hymns for public worship
- Hymns of trial or calamity
- Messianic hymns
- General religious hymns
SmithCreekMusic.com elaborates on the meaning of "hymn" by defining it pragmatically as a reverent, devotional song that expresses "God's purposes" in life or a worshiper's "attitude toward God":
[A hymn is] reverently and devotionally conceived, which is designed to be sung and which expresses the worshipper's attitude toward God, or God's purposes in human life. (smithcreekmusic.com)