i am currently reading about memory system in computer architecture. I wanted to know, Is the word length the number of bits input to the ALU?
'Word length' is a fairly loose concept whose meaning has varied somewhat over the course of computer design history, depending on what was important, or what was seen as a performance limitation at the time.
Unfortunately, in your question, you've used the terms 'memory system', 'computer architecture', and 'ALU'. Each can define word length, and each can be different, though they often will align.
The most common measure of word length the width of the internal data bus and multiplexers, so the registers and most importantly the ALU. The original 4004 and 8085 were thus 4 and 8 bit machines respectively. The addressable memory on these machines was much more than that, through paging, and use of double-width registers. This was continued into the 16 bit 8086 which could address a 20 bit memory space. Once properly addressable linear memory was seen as important, the move from 32 bit to 64 bit 'computers' (operating systems) was based on addressable memory space, even though by now internal registers were a mix of 32, 64, 80, 128 bits, and more.