Tax evasion is a potential criminal charge. Tax avoidance is perfectly acceptable.
26 U.S.C. § 7201 defines what is tax evasion. It states in relevant part that any person who willfully attempts in any manner to evade or defeat any tax imposed by this title or the payment thereof shall, in addition to other penalties provided by law, be guilty of a felony and, upon conviction thereof, shall be fined not more than $100,000 ($500,000 in the case of a corporation), or imprisoned not more than 5 years, or both, together with the costs of prosecution.
Federal prosecutors might also add additional charges beyond tax evasion. Those charges could include conspiracy charges, under 18 U.S.C. § 371, or tax fraud under 26 U.S.C. § 7206. The United States Sentencing Commission (USSC) recently released its Tax Fraud Report for fiscal year 2023. It states that off the 64,124 cases reported to the Commission in fiscal year 2023, 363 involved tax fraud (down 27% since FY 2019). It is unclear if this downward trend will continue.