"Nondeductible contributions" refers to contributions or donations that cannot be subtracted or deducted from your taxable income when calculating the amount of taxes you owe. Full definition
Subject to various limitations, 401k plans and other retirement plans maintained by employers can offer the opportunity to make nondeductible contributions instead of, or in addition to, deductible contributions. (fairmark.com)
Some people have traditional IRAs that consist mostly of nondeductible contributions. (fairmark.com)
There's no income limit on nondeductible contributions. (traditionalira.com)