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Snippet from Wikipedia: Reserved word

In a programming language, a reserved word (sometimes known as a reserved identifier) is a word that cannot be used by a programmer as an identifier, such as the name of a variable, function, or label – it is "reserved from use". In brief, an identifier starts with a letter, which is followed by any sequence of letters and digits (in some languages underline '_' is treated as a letter!).

In an imperative programming language and in many object-oriented programming languages, apart from assignments and subroutine calls, keywords are often used to identify a particular statement, e.g. if, while, do, for, etc. Many languages treat keywords as reserved words, including Ada, C, C++, COBOL, Java, and Pascal. The number of reserved words varies widely from one language to another: C has about 30 while COBOL has about 400.

Note that a few languages do not have any reserved words. Fortran and PL/I identify keywords by context, while Algol 60 and Algol 68 generally use stropping to distinguish keywords from programmer-defined identifiers, e.g. .if or 'if or 'if'is a keyword distinct from identifier if.

Most programming languages have a standard library (or libraries), e.g. mathematical functions sin, cos, etc. The names provided by a library are not reserved, and can be redefined by a programmer if the library functionality is not required.

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• BASIC LET - assigns a value (which may be the result of an expression) to a variable. In most dialects of BASIC, LET is optional, and a line with no other identifiable keyword will assume the keyword to be LET.

• BASIC DATA - holds a list of values which are assigned sequentially using the READ command.

• BASIC READ - reads a value from a DATA statement and assigns it to a variable. An internal pointer keeps track of the last DATA element that was read and moves it one position forward with each READ. Most dialects allow multiple variables as parameters, reading several values in a single operation.

• BASIC RESTORE - resets the internal pointer to the first DATA statement, allowing the program to begin READing from the first value. Many dialects allow an optional line number or ordinal value to allow the pointer to be reset to a selected location.

• BASIC DIM - Sets up an array.

• BASIC IF ... THEN ... {ELSE} - used to perform comparisons or make decisions. Early dialects only allowed a line number after the THEN, but later versions allowed any valid statement to follow. ELSE was not widely supported, especially in earlier versions.

• BASIC FOR ... TO ... {STEP} ... NEXT - repeat a section of code a given number of times. A variable that acts as a counter, the “index”, is available within the loop.

• BASIC WHILE ... WEND and REPEAT ... UNTIL - repeat a section of code while the specified condition is true. The condition may be evaluated before each iteration of the loop, or after. Both of these commands are found mostly in later dialects.

• BASIC DO ... LOOP {WHILE} or {UNTIL} - repeat a section of code indefinitely or while/until the specified condition is true. The condition may be evaluated before each iteration of the loop, or after. Similar to WHILE, these keywords are mostly found in later dialects.

• BASIC GOTO - jumps to a numbered or labelled line in the program. Most dialects also allowed the form GO TO.

• BASIC GOSUB ... RETURN - jumps to a numbered or labelled line, executes the code it finds there until it reaches a RETURN command, on which it jumps back to the statement following the GOSUB, either after a colon, or on the next line. This is used to implement subroutines.

• BASIC ON ... GOTO/GOSUB - chooses where to jump based on the specified conditions. See Switch statement for other forms.

• BASIC DEF FN - a pair of keywords introduced in the early 1960s to define functions. The original BASIC functions were modelled on FORTRAN single-line functions. BASIC functions were one expression with variable arguments, rather than subroutines, with a syntax on the model of DEF FND(x) = x*x at the beginning of a program. Function names were originally restricted to FN, plus one letter, i.e., FNA, FNB …

• BASIC LIST - displays the full source code of the current program.

• BASIC PRINT - displays a message on the screen or other output device.

• BASIC INPUT - asks the user to enter the value of a variable. The statement may include a prompt message.

• BASIC TAB - used with PRINT to set the position where the next character will be shown on the screen or printed on paper. AT is an alternative form.

• BASIC SPC - prints out a number of space characters. Similar in concept to TAB but moves by a number of additional spaces from the current column rather than moving to a specified column.

• BASIC ABS - Absolute value

• BASIC ATN - Arctangent (result in radians)

• BASIC COS - Cosine (argument in radians)

• BASIC EXP - Exponential function

• BASIC EXPINT - Integer part (typically floor function)

• BASIC EXPLOG - Natural logarithm

• BASIC EXPRND - Random number generation

• BASIC EXPSIN - Sine (argument in radians)

• BASIC EXPSQR - Square root * [[BASIC EXPTAN - Tangent (argument in radians)

• BASIC REM - holds a programmer's comment or REMark; often used to give a title to the program and to help identify the purpose of a given section of code.

• BASIC USR - transfers program control to a machine language subroutine, usually entered as an alphanumeric string or in a list of DATA statements.

• BASIC CALL -

alternative form of BASIC USR found in some BASIC dialects. Does not require an artificial parameter to complete the function-like syntax of USR, and has a clearly defined method of calling different routines in memory.

• BASIC TRON / * BASIC TROFF - turns on display of each line number as it is run (“TRace ON”). This was useful for debugging or correcting of problems in a program. TROFF turns it back off again.

• BASIC ASM - some BASIC compilers such as Freebasic, [37] Purebasic, [38] and Powerbasic [39] also support inline assembly language, allowing the programmer to intermix high-level and low-level code, typically prefixed with “ASM” or “!” statements.